Meaning: 'Reaction' refers to the way someone responds to something, in this case, how quickly drivers can respond to obstacles while driving.

Wrong options:

• early: The word 'early' is an adjective that describes timing but does not fit the context of measuring response times, which requires a noun.
• medieval: 'Medieval' is an adjective that refers to a historical period and is irrelevant in this context, as it does not relate to driving or response times.
• reasonable: 'Reasonable' is an adjective that describes something as fair or sensible, which does not apply to the context of measuring how quickly drivers react.

Correct answer: crash

Explanation: The word 'crash' is a verb that accurately describes the action of colliding with an obstacle, fitting both grammatically and contextually within the sentence about driving and obstacles.

Meaning: 'Crash' means to collide violently with something, which is the outcome being investigated in relation to reaction times and heartbeats.

Wrong options:

• slash: The word 'slash' is a verb that implies cutting or striking, which does not relate to the context of driving and collisions.
• splash: 'Splash' is a verb that typically refers to water or liquid, making it irrelevant in the context of a driving scenario where a collision occurs.
• split: 'Split' is a verb that means to divide or separate, which does not convey the intended meaning of colliding with an obstacle.

Correct answer: aroused

Explanation: The word 'aroused' is an adjective that describes a heightened state of alertness or excitement, which fits grammatically and contextually as it relates to the physiological state of the driver during high-stress situations.

Meaning: 'Aroused' means to be in a state of heightened emotional or physical alertness, which can affect driving performance.

Wrong options:

• technical: The word 'technical' is an adjective that refers to specialized knowledge or skills, which does not apply to the emotional state of a driver.
• flexible: 'Flexible' is an adjective that describes the ability to bend or adapt, which is not relevant to the context of emotional or physiological states while driving.
• educated: 'Educated' is an adjective that refers to someone who has received education, which does not relate to the emotional or physiological state affecting driving.