**Adjective Definition:**
An adjective is a word that describes or modifies a noun or pronoun. It provides more information about the noun or pronoun by answering questions such as what kind, which one, how many, or how much.

**Explanation:**

Out of the given options, the adjective is:

**d) Jumpy**

Let's analyze each option to understand why only "Jumpy" is an adjective:

a) Jump: This is a verb, not an adjective. It describes an action of moving off the ground.

b) Jumping: This is a present participle form of the verb "jump." Like "jump," it is also a verb, not an adjective. It describes an ongoing action of moving off the ground.

c) Jumps: This is the third-person singular present tense form of the verb "jump." Similar to the previous options, it is a verb and not an adjective.

d) Jumpy: This is the correct answer. It is an adjective that describes someone or something that is nervous, easily startled, or prone to sudden movements. It modifies a noun by providing more information about its characteristics.

In summary, "Jumpy" is the only option that functions as an adjective because it describes or modifies a noun or pronoun, while the other options are verbs.