The assertion that the Gandhara School of art exhibits Hellenistic influences is true. Gandhara art, which emerged in present-day Pakistan and Afghanistan, incorporates a blend of Indian native, Persian, and Greco-Roman artistic influences. This style flourished during the period of Greco-Bactrian rule and under the Kushan Empire, and often depicted Buddhist themes.

However, the reason given — that the Hinayana form was influenced by Gandhara art — is not accurate. Hinayana, often referred to as Theravada, is a branch of Buddhism, not a form of art. While there may have been some mutual influence between Gandhara art and Buddhist thought, it`s not accurate to claim that the Hinayana form of Buddhism was directly influenced by Gandhara art. Thus, option-3 "A is true, but R is false" is the correct answer.