Meaning: 'Adapted' means modified or adjusted to fit different needs or circumstances.

Wrong options:

• solved: 'Solved' is a past participle that implies resolution of a problem, which does not fit the context of describing research fields.
• tangle: 'Tangle' is a noun or verb that refers to a complicated situation, which does not relate to the adaptability of research fields.
• require: 'Require' is a verb that indicates necessity, which does not fit as it does not describe the nature of research fields.
• regard: 'Regard' is a noun or verb that refers to consideration or respect, which does not apply to the adaptability of research fields.
• set: 'Set' is a noun or verb that refers to a collection or to place something, which does not convey the idea of adaptability.
• observer: 'Observer' is a noun that refers to someone who watches or notices something, which does not fit the context of describing research fields.

Correct answer: require

Explanation: The term 'require' is appropriate as it indicates that certain phenomena in geology need a high level of intelligence and knowledge for investigation. This fits grammatically and contextually.

Meaning: 'Require' means to need something for a particular purpose.

Wrong options:

• solved: 'Solved' does not fit here as it implies a completed action rather than a necessity for investigation.
• tangle: 'Tangle' does not relate to the context of needing intelligence; it refers to a complicated situation.
• adapted: 'Adapted' does not fit as it describes suitability rather than necessity in this context.
• regard: 'Regard' does not apply here as it refers to consideration rather than a requirement for investigation.
• set: 'Set' does not convey the idea of necessity and is unrelated to the context of geological phenomena.
• observer: 'Observer' does not fit as it refers to a person watching rather than a requirement for understanding phenomena.

Correct answer: solved

Explanation: The word 'solved' fits grammatically as it indicates that many problems in geology can be addressed or resolved by common people. This aligns with the context of accessibility in geology.

Meaning: 'Solved' means to find an answer or solution to a problem.

Wrong options:

• require: 'Require' does not fit as it indicates necessity rather than resolution of problems.
• tangle: 'Tangle' does not relate to the context of resolving problems; it refers to a complicated situation.
• adapted: 'Adapted' does not fit as it describes suitability rather than resolution of problems.
• regard: 'Regard' does not apply here as it refers to consideration rather than solving problems.
• set: 'Set' does not convey the idea of resolution and is unrelated to the context of solving geological problems.
• observer: 'Observer' does not fit as it refers to a person watching rather than the act of solving problems.

Correct answer: observer

Explanation: The term 'observer' fits grammatically and contextually as it refers to someone who notices or learns about geological facts, aligning with the idea of casual engagement with geology.

Meaning: 'Observer' refers to a person who watches or pays attention to something.

Wrong options:

• solved: 'Solved' does not fit as it refers to resolution rather than a person observing facts.
• tangle: 'Tangle' does not relate to the context of observing; it refers to a complicated situation.
• require: 'Require' does not fit as it indicates necessity rather than a role in observing facts.
• adapted: 'Adapted' does not apply here as it describes suitability rather than a person observing.
• set: 'Set' does not convey the idea of a person and is unrelated to the context of observing geological facts.