Folic acid is a nutrient present in leafy vegetables and some other

B. Folic acid cannot be synthesized or stored by the body.

This statement provides support for the argument, as it implies that individuals need to obtain folic acid from their diet regularly. If folic acid cannot be stored, then a decrease in dietary folic acid would indeed lead to lower blood levels of folic acid over time. This strengthens the argument that a decrease in dietary folic acid could increase the risk of a heart attack

Premise:
       A.  Leafy veggies etc. etc contains folic acid
       B.  High folic Acid in Blood means low risk of Heart attack
Conclusion:
       Diet rich in folic acid means low risk of Heart Attack

Prethinking : Unstated assumption -> what causes the jump from folic acid in diets to folic acid in blood ?
                                                      does eating folic acid rich veggies ensure folic acid rich blood ?


A. Many of the people who had heart attacks during the study were eating a high-fat diet, which is known to contribute to the formation of arterial blockages.
It says high fat diet causes low heart attack - no relation to our passage ( irrelevant )

B. Folic acid cannot be synthesized or stored by the body.
It gives an hint that folic acid cannot come from anyother source ( produced in body / stored ) so they must come from the diets - Properly address the unstated assumption hence our answer

C. Arterial blockages usually take years to develop.
It talks about time frame not anything about folic acid sources

D. The group that had the fewest heart attacks also had the best exercise habits.
It gives an alternate reason for heart attack prevention instead of strengthening the unstated assumption

E. The subjects in the study were tested for folic acid levels most frequently in the first two years of the study.
Frequency of testing is irrelevant in our passage

The passage talks about a certain nutrient, folic acid, that is in leafy vegetables and some other foods. That it helps stop blockages that cause heart attacks and that a study showed that people who had a lot of this nutrient did not have as many heart attacks. And thus why a decrease in DIETARY folic acid would increase the chances of having a heart attack.

What could strengthen this?

B. Folic acid cannot be synthesized or stored by the body.

Might be difficult to see why (B) is the answer but the word DIETARY may be worth looking at. Basically, if a person can mainly only get this special nutrient from certain vegetables for example - and cannot internally create or store it - it helps support why having less DIETARY FOLIC ACID would increase the chances of a heart attack. In a sense, a person need to be constantly eating these vegetables to keep your folic acid levels from falling.

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Premises:­
Folic acid is a nutrient present in leafy vegetables and some other foods.
It inhibits the formation of substances found in arterial blockages that cause heart attacks.
In a study, people with the highest blood levels of folic acid had fewer heart attacks than those with the lowest levels.

Conclusion: A decrease in dietary folic acid will increase the risk of a heart attack.

Important to note here before you proceed - the conclusion says "a decrease in dietary folic acid will increase the risk of a heart attack." It doesn't say "a decrease in folic acid will increase the risk of a heart attack." which is what we would have expected. 

What would strengthen this claim? We need to strengthen that if one does not consume folic acid in food, one's risk of heart attack increases. 

A. Many of the people who had heart attacks during the study were eating a high-fat diet, which is known to contribute to the formation of arterial blockages.

This option makes us doubt the study that the author is using to support his statement. It says that the reason for heart attack in those people was different. It certainly does not strengthen the conclusion about the impact of folic acid. 

B. Folic acid cannot be synthesized or stored by the body.

This is interesting. It tells us that folic acid cannot be created or stored by the body. This means that one must consume dietary folic acid. It strengthens the "dietary" part of the claim. 

C. Arterial blockages usually take years to develop.

Irrelevant.

D. The group that had the fewest heart attacks also had the best exercise habits.

Again, this makes us doubt the study that the author is using to support his statement. It says that the reason for fewer heart attacks in those people was exercise, not folic acid. Hence it doesn't strengthen the conclusion.

E. The subjects in the study were tested for folic acid levels most frequently in the first two years of the study.

Frequency of testing is irrelevant. 

Answer (B)