**Core Concept**
The question is testing the association between a specific amino acid, l-tryptophan, and a neurotransmitter. l-Tryptophan is an essential amino acid that serves as a precursor to several important compounds in the brain, including serotonin.

**Why the Correct Answer is Right**
The ingestion of l-tryptophan increases its availability in the brain, where it is converted into serotonin through a series of enzyme-catalyzed reactions. This process is facilitated by the enzyme tryptophan hydroxylase, which is present in the neurons of the raphe nucleus. Serotonin is then released from the presynaptic neuron and binds to its receptors on the postsynaptic neuron, producing various physiological effects, including mood regulation and appetite control.

**Why Each Wrong Option is Incorrect**
**Option A:** This is incorrect because l-tryptophan is not directly converted into dopamine. While both serotonin and dopamine are involved in mood regulation, they are produced from different precursor amino acids: l-tryptophan for serotonin and l-tyrosine for dopamine.
**Option B:** This is incorrect because l-tryptophan is not converted into acetylcholine. Acetylcholine is produced from the amino acid choline, not tryptophan.
**Option C:** This is incorrect because l-tryptophan is not converted into GABA (gamma-aminobutyric acid). GABA is produced from the amino acid glutamate through the action of the enzyme glutamic acid decarboxylase.
**Option D:** This is incorrect because l-tryptophan is not converted into norepinephrine. Norepinephrine is produced from the amino acid tyrosine through a series of enzyme-catalyzed reactions.

**Clinical Pearl / High-Yield Fact**
It's worth noting that the relationship between l-tryptophan and serotonin is complex and can be influenced by various factors, including dietary intake, genetic variations, and certain medications. For example, some antidepressant medications, such as selective serotonin reuptake inhibitors (SSRIs), work by increasing the levels of serotonin in the brain by inhibiting its reuptake.

**Correct Answer: B. Serotonin**