Meaning: 'Appealing' means attractive or interesting, suggesting that the smell can be enticing to certain creatures, potentially serving as a defense mechanism.

Wrong options:

• agreeable: 'Agreeable' implies a pleasant quality but is less specific to the context of a smell compared to 'appealing.'
• enchanting: 'Enchanting' suggests a magical or captivating quality, which is not the intended meaning in this context of defense.
• ordinary: 'Ordinary' means common or unremarkable, which contradicts the idea of a defense mechanism that should stand out.

Correct answer: struggle

Explanation: The word 'struggle' is the most appropriate choice as it refers to the fight for survival, fitting the context of the sentence. It is a noun that conveys the ongoing effort of animals to survive, aligning with the preceding discussion of defenses.

Meaning: 'Struggle' means a fight or effort against difficulties, particularly in the context of survival in nature.

Wrong options:

• march: 'March' implies a steady walk or advance, which does not relate to the concept of survival efforts.
• game: 'Game' refers to hunted animals or a type of sport, which does not fit the context of survival struggles.
• campaign: 'Campaign' suggests a planned series of activities, which is not suitable for the instinctual nature of survival.

Correct answer: shapes

Explanation: The word 'shapes' is the correct choice as it indicates the influence of evolution on an animal's actions and behaviors. It is a verb that aligns with the subject 'evolution,' showing an active role in determining characteristics.

Meaning: 'Shapes' means to form or influence something, indicating that evolution plays a crucial role in determining behaviors.

Wrong options:

• pieces: 'Pieces' refers to parts of something, which does not fit grammatically or contextually in this sentence.
• features: 'Features' implies characteristics but does not convey the active influence that 'shapes' does in this context.
• aspects: 'Aspects' refers to different parts or features, which lacks the dynamic quality of influence implied by 'shapes.'

Correct answer: automatic

Explanation: The word 'automatic' is appropriate as it describes behaviors that occur instinctively without conscious thought. It fits grammatically and contextually, emphasizing the nature of these behaviors as inherent and not learned.

Meaning: 'Automatic' means occurring without conscious thought or effort, indicating that these behaviors are instinctual.

Wrong options:

• dangerous: 'Dangerous' implies a risk or threat, which does not accurately describe the nature of instinctive behaviors.
• automatic: 'Automatic' implies a lack of conscious effort, which is the intended meaning here, making it the correct choice.
• difficult: 'Difficult' suggests a challenge, which contradicts the idea of instinctive behaviors being easy or inherent.
• ascetic: 'Ascetic' refers to a lifestyle characterized by abstinence, which is unrelated to the context of instinctive behaviors.

Correct answer: trial

Explanation: The word 'trial' fits well as it refers to the process of testing or experimenting, which aligns with the concept of learning through experience. It is a noun that complements the phrase 'by trial and error,' indicating a method of learning.

Meaning: 'Trial' means an attempt or test, particularly in the context of learning through experimentation.

Wrong options:

• attempt: 'Attempt' is a synonym but does not fit the idiomatic expression 'trial and error' as effectively as 'trial' does.
• doing: 'Doing' is a gerund that does not fit grammatically in this context, as it lacks the specific meaning of testing or experimenting.
• tasting: 'Tasting' refers specifically to the sense of taste, which is not relevant to the broader concept of learning through experience.