The hormone that delays Arabidopsis seed germination on MS medium is abscisic acid (ABA), so the correct option is (4) Abscisic acid. ABA is the primary dormancy‑inducing, germination‑inhibiting hormone in seeds, whereas the others do not classically cause strong delay of germination.

Core concept: ABA vs germination

Abscisic acid accumulates during seed maturation and maintains seed dormancy, preventing premature germination. In Arabidopsis and many other plants, exogenous ABA added to germination medium slows or blocks radicle emergence, so seeds on ABA‑containing MS plates germinate later than controls. Removing or lowering ABA, or increasing gibberellin, breaks dormancy and accelerates germination.

Option‑by‑option explanation

1. Jasmonic acid
   Jasmonic acid participates mainly in defense and stress responses. At high levels it can inhibit germination, but it is not the principal hormone controlling “normal” seed dormancy in Arabidopsis. Exam questions that describe delayed germination almost always point to ABA rather than jasmonate.

2. Cytokinin
   Cytokinins promote cell division and shoot development. They generally do not act as strong dormancy hormones; in some contexts they can even support germination or seedling growth. Adding cytokinin alone to MS medium would not typically cause marked delay in seed germination.

3. Auxin
   Auxin regulates cell elongation, tropisms and root initiation. Normal auxin levels do not keep seeds dormant, and auxin is not considered the primary inhibitor of germination. Excess auxin can perturb growth, but it is not the classic cause of delayed germination used in such problems.

4. Abscisic acid (ABA) – Correct
   ABA is the central hormone for inducing and maintaining seed dormancy in Arabidopsis. Exogenous ABA on MS plates mimics or enhances endogenous ABA, so seeds either fail to germinate or germinate much later than on hormone‑free medium. Therefore, the late germination observed in the question is best explained by the presence of ABA.

SEO‑oriented introduction (for article use)

When Arabidopsis thaliana seeds are sown on Murashige and Skoog (MS) medium, they normally germinate quickly unless a dormancy‑promoting hormone is present. Abscisic acid is the key hormone that maintains seed dormancy and directly inhibits radicle emergence, so adding ABA to MS medium delays seed germination, making option (4) Abscisic acid the correct answer for this plant hormone MCQ.