Fill in the blank with the most appropriate adjective to complete the sentence meaningfully: We acted on a __________ impulse.

Introduction / Context:
This vocabulary question tests your ability to choose an adjective that fits both the grammatical structure and the meaning of the sentence. The sentence describes acting on an impulse, which suggests a sudden, short lived feeling or desire. Understanding the subtle differences between words that share the same root can greatly improve accuracy in English usage questions.

Given Data / Assumptions:

• The sentence is "We acted on a __________ impulse."
• All four options are adjectives: "momentary", "momentous", "memorable", and "meritorious".
• The noun "impulse" refers to a sudden desire or urge to act, often without deep thought.
• We must select one adjective that best describes the nature of such an impulse.

Concept / Approach:
The key idea here is collocation and meaning. The correct adjective should describe the impulse itself, matching both common usage and logical sense. "Impulse" is generally associated with something sudden and brief, not something great in importance, worthy, or easy to remember. So we must pick the adjective that suggests a short duration rather than importance, quality, or memorability.

Step-by-Step Solution:
Step 1: Consider the meaning of "momentary". It means lasting for a very short time or brief.Step 2: Combine it with the noun: "a momentary impulse" describes an urge that appears suddenly and disappears just as quickly, which is exactly how an impulse behaves.Step 3: Consider the meaning of "momentous". It means very important or of great significance, for example a "momentous decision". It does not describe short duration.Step 4: Consider "memorable". This means worth remembering, usually because something is special or impressive, which does not logically describe an impulse itself.Step 5: Consider "meritorious". This means deserving praise or reward. An impulse may or may not be meritorious, but that is not the usual way we talk about impulses.Step 6: The only adjective that naturally and frequently collocates with "impulse" is "momentary", so option A is correct.

Verification / Alternative check:
We can verify by checking real world usage in standard English: phrases like "a momentary impulse", "a momentary hesitation", and "a momentary lapse" are common. By contrast, phrases such as "a momentous impulse" or "a meritorious impulse" sound strange and are rarely, if ever, used. "Memorable impulse" also does not fit typical patterns. This confirms that "momentary" is the best and most natural choice.

Why Other Options Are Wrong:
Option B, "momentous", expresses importance rather than brevity. It would be better used with nouns like "occasion", "event", or "decision", not "impulse".
Option C, "memorable", focuses on something that is worth remembering, such as a "memorable day" or "memorable speech". An impulse is a fleeting internal feeling, not an event that is usually labelled this way.
Option D, "meritorious", is used for actions or achievements that deserve praise, for example "meritorious service". It does not naturally qualify the noun "impulse" in standard collocation.

Common Pitfalls:
Students sometimes confuse "momentary" and "momentous" because both share the same root "moment". They may think that "momentous" also means short lived. However, "momentary" refers to duration, while "momentous" refers to great importance. Similarly, test takers may be attracted to positive sounding words like "memorable" and "meritorious" without checking whether they really fit the noun in the sentence.

Final Answer:
The correct word is "momentary", so the answer is option A.