This allows the conclusion to be validly drawn. If, within milk, an elderly person is given everything that is needed to absorb the calcium that they need, as well as the calcium itself (guaranteed in the premises), then some elderly people can lower their blood pressure by drinking milk.

This is a strengthening answer choice, but it is not a sufficient assumption. (B) does not guarantee the conclusion because it is possible that milk does not have these unwanted substances, but still does not have enough vitamin D to allow elderly people to absorb the calcium.

This is a necessary assumption, but it is not a sufficient assumption. We do need for (C) to be true if the argument stands a chance of becoming valid (aka a necessary assumption), but (C) does not guarantee the argument to be valid (aka a sufficient assumption).

We don’t know anything about normal blood pressure. “Normal,” as it is used in (D), is absolute, but blood pressure being raised or lowered, as used in the stimulus, is relative. We can’t assume that, just because the blood pressure of these elderly people has raised, it is abnormal, and we can’t assume that if they successfully lower their blood pressure it will be normal.

This does not allow us to conclude that milk will solve the issue of calcium deficiency in a way that will lower blood pressure. (E) takes something that we know to be true about some older people (vitamin D deficiency led to calcium deficiency) and makes it a conditional rule.