Typical rogue element approach for this question.
Conclusion: sugars can play a major role in mood elevation, helping one to feel relaxed and anxiety-free.
Premise: Tryptophan is transformed into serotonin in the brain causing an increase in the brain's serotonin level.

Fix the missing pieces in the jigsaw puzzle.

Assumption: Serotonin causes mood elevation.

Now lets look at the options:

(A) Elevation of mood and freedom from anxiety require increasing the level of serotonin in the brain. [This is the reverse. Increased level of serotonin causes mood elevation is what our assumption says, but elevation in mood does not necessarily require increased serotonin activity. Say my mood is elevated when listening to music. So is serotonin involved?? NO. This is a nice trap laid to lure test takers. I hope i have differentiated the subtle difference in the meaning. Eliminated!]

(B) Failure to consume foods rich in sugars results in anxiety and a lowering of mood. [we know what happens when we consume sugar rich foods. But what happens when we dont consume is beyond the scope of our stimulus.- remember when we say animals exposed to sunlight suffer from skin cancer we cannot infer that animals not exposed to sunlight do not suffer from skin cancer. Eliminated!]

(C) Serotonin can be produced naturally only if tryptophan is present in the bloodstream. [we do not have info to comment on the various methods of serotonin production. we know tryptophan transforms in serotonin. There may be other ways too. Eliminated!]

(D) Increasing the level of serotonin in the brain promotes relaxation and freedom from anxiety. [Bingo! we have our assumption stated word to word. click. confirm and move. ]

(E) The consumption of protein-rich foods results in anxiety and a lowering of mood. [This is exact opposite to what is stated in the stimulus. Eliminated!]

Hope this helps!