Smoking is a known cause of certain serious health problems, including emphysema and lung cancer. Now, an additional concern can be added to the list of maladies caused by smoking. A recent study surveyed both smokers and nonsmokers, and found that smokers are significantly more anxious and nervous than nonsmokers.

Which of the following is an assumption on which the argument rests?
(a) Anxiety and nervousness can lead to serious health problems.
(b) Anxiety and nervousness do not make individuals more likely to start smoking.
(c) Equivalent numbers of smokers and nonsmokers were surveyed for the study.
(d) Smokers are aware of the various health problems attributed to smoking, including lung cancer and emphysema.
(e) Smokers who had smoked a cigarette immediately before responding to the survey were more anxious and nervous than smokers who had not smoked for several hours.

according to the MGMAT answer is B. what is the correct answer if the question is which of the below statements strengthen above reasoning?
Cant we think this question as strengthn question "Which of the following is an assumption on which the argument rests?"

If question asked is which of the below statement infered from the above?
Then the answer is A ???

Assumption questions can sometimes be thought of as strengthen questions, but that doesn't always work. In this case, yes, the correct answer does strengthen the argument's conclusion.

None of the answer choices would work as the correct answer to an inference question. A correct "inference" answer does not actually infer the way we infer in the real world. Instead, the correct answer to an inference question has to be 100% true based upon the information presented - essentially just restating something the argument (or passage) said in a different way.

For example, if I told you that my favorite flavor of ice cream is chocolate, a correct GMAT inference might be "Vanilla is not my favorite flavor of ice cream." Slightly different take on the info, but you can't argue with it - if one flavor of ice cream is my favorite then, by definition, another flavor is not.

Isn't it safe to say that all assumptions in an argument are also inferences because when looking at an inference question, we presuppose all statements in the stimulus are true?

So an assumption, which by definition is required to be true for the argument to be true, is also an inference because all statements in the passage (hence the argument) are taken to be true. Basically, if the assumption has to be true for the argument to be true; if the argument is true, then the assumption must be true (hence an inference).

For example, if argument says A caused B. An assumption can be that C did not cause B. But that is also an inference.

I think I follow what you're saying: assumptions and inferences are similar in that they are not explicitly stated in the argument and they must be true in order for the argument to have integrity. But I'm not clear how these facts could be helpful in answering GMAT assumption and inference questions. They require very different approaches.

On assumption problems, you're looking for a truth upon which the conclusion absolutely depends. So, you're uncovering something that was already "baked into" the argument.

On inference problems, you draw an air-tight conclusion that follows from the premises. Here, you're extending the argument out into a new truth.

Again, is it safe to say "All assumptions can also be inferences"? (obviously, all inferences cannot be assumptions).