Passage Analysis
- Conclusion/Claim: Caffeine must be as beneficial for weight loss as exercise
- Logic: Consumption of caffeine stimulates the metabolism in almost the same way as physical exercise

Pre-Thinking
In what scenario would caffeine not be as beneficial for weight loss as exercise?
Given: Consumption of caffeine stimulates the metabolism in almost the same way as physical exercise

What if metabolism is not a proper indicator of weight loss? i.e. what if the effect an activity (caffeine, exercise) has on metabolism does not translate to effect of that activity on weight loss?

The author is assuming that -
The impact an activity has on metabolism is an accurate measure of the impact the activity will have on weight loss.

Option Choice Analysis
A. The body’s short-term response to any stimulant is an effective measure of that stimulant’s long-term effect on the body.
Out of scope - short term or long term response is not anywhere covered in the argument - it cannot be the assumption based on which the author makes the claim

B. The consumption of caffeine is an effective way for a person to control his or her weight.
Out of scope. The argument is about the comparison between caffeine and exercise on how they fare with regard to weight loss. Effectiveness of caffeine does not impact this argument.

C. Caffeine has no negative side effects for a person’s health.
Irrelevant.

D. The effect an activity has on the body’s metabolism level is an adequate measure of how that activity will affect weight loss.
Correct, and in line with our analysis. The the impact in metabolism does not translate to an equivalent impact on weight loss, this argument breaks down. This is the correct choice.

E. A person who consumes caffeine is unlikely to supplement that consumption with physical exercise or other methods of weight loss.
This tells us that any weight loss in a person using caffeine can be attributed to caffeine, and not to exercise of any other method.

Let us try negation.
Negation: A person who consumes caffeine is likely to supplement that consumption with physical exercise/other methods. This does not necessarily break the conclusion.

Why? - Say a weight loss of 10 kgs is observed in a person who consumes caffeine and supplements with physical exercise.
It is possible that 5 kgs of weight loss can be attirbuted to caffeine only, and 5 kgs due to exercise only. What this means is that both caffeine and exercise were equally beneficial. Hence, the conclusion can still hold true.

So, this option, if negated does not definitely break the conclusion. Hence, it is not the correct choice.

Hope this helps.