The hormone involved in the regulation of florigen synthesis is

The correct answer is gibberellin.

Explanation:
Florigen is a systemic signal (often associated with the FT protein) that initiates flowering in plants. The synthesis and activation of florigen are regulated by various environmental cues and internal hormonal signals. Among the plant hormones listed, gibberellin plays a crucial role in promoting flowering, particularly in long-day plants and those requiring vernalization (cold treatment) to flower. It acts by regulating the expression of genes involved in the florigen synthesis pathway, thereby inducing the transition from vegetative to reproductive growth under non-inductive conditions.

Let's break down why the other options are not correct in this context:
1. Zeatin: A type of cytokinin, mainly involved in cell division and shoot formation, but is not the primary hormone regulating florigen synthesis.
2. Cytokinin: Generally promotes cell division and lateral bud growth, but does not serve as the primary hormonal regulator for the synthesis of the florigen signal.
3. Ethylene: A gaseous hormone involved in fruit ripening, senescence, and abscission, and typically acts to inhibit or modify flowering depending on the species, rather than promoting florigen synthesis.