Not necessary, because because even if increasing serotonin is NOT REQUIRED to elevate one’s mood, it can still causally contribute to elevation of mood. That’s all the author is committed to — that serotonin causally contributes to mood elevation.

Not necessary, because although the author believes eating sugars can help one’s mood, that doesn’t imply that failing to eat sugars lowers one’s mood. Failing to eat sugars can simply leave one’s mood unchanged.

Not necessary, because although we know that tryptophan transforms into serotonin, that doesn’t imply that serotonin requires tryptophan in order to be formed. Maybe there are other chemicals that can also turn into serotonin. That doesn’t change the fact that we know tryptophan is one chemical that transforms into serotonin.

Necessary, because this is the final link in the causal chain between sugars and mood elevation. If (D) were not true, then we’d have no reason to think that sugars can lead to mood elevation.

Not necessary, because the causal chain begins by eating sugars after eating proteins. But if one merely eats proteins, without eating sugars, the author does not tell us what happens, and the conclusion doesn’t concern what happens if sugar isn’t ingested after eating proteins.