The neurotransmitters most likely implicated in generalized anxiety disorder (GAD) are GABA (Gamma-Aminobutyric Acid) and serotonin. GABA is an inhibitory neurotransmitter that helps regulate anxiety, while serotonin plays a role in mood regulation and feelings of well-being. Although dopamine and norepinephrine are also involved in anxiety regulation, they are not the primary neurotransmitters implicated in GAD. Therefore, the correct answers are a. GABA and c. Serotonin.