I think option E is the correct answer as remaining options are irrelavant and out of scope

E is correct. As remaining options are not relevent to the assumption.

The passage suggests that the parents give their children a diet heavy on fish, meat and eggs.

Now we are asked to find an assumption. Think that WHY it was suggested to include fish, meat and eggs?

Because the current food they are eating does not have enough protein.

C states the same and hence the answer.

Shouldn't the answer be D here as it says the the protein should be easy to absorb for teenagers for them to grow in height.

D are information that we do already know. It is not our assumption

Be careful on this

Even if this is not a standard ETS question, what would the answer be in your opinion? Carcass

Explanation

The argument's core logic is:

Teenagers need a lot of protein that is easily absorbed to grow.
Therefore, parents should give them a diet heavy on fish, meat, and eggs.


This conclusion only works if we assume that the alternative-the "saladism" fad-does not provide enough of that easily absorbed protein. The author is implicitly dismissing salads as a sufficient source, so the conclusion that fish/meat/eggs are necessary rests on that assumption.

Why the other options are incorrect:

(A) Saladism is bad for children. This is too broad. The argument only needs to assume that saladism is inadequate in protein, not that it is entirely "bad" in every way. This overstates the assumption.
(B) It is possible for many adults to adopt a practice that has no logical reasons to be adopted. This is completely irrelevant to the teenage nutrition argument.
(D) The protein that are harder to absorb and imbibe do not help a teenager to grow in height. The argument says teenagers need protein that is easy to absorb-it does not claim that hard-to-absorb protein is useless. The conclusion does not rely on ruling out harder-to-absorb proteins entirely.
(E) Fish, meat and eggs contain protein of different sorts. The argument does not require that these foods contain different proteins-only that they contain the right kind (easily absorbed) in sufficient amounts.