Adjective:
An adjective is a word that describes or modifies a noun or pronoun. It provides more information about the noun or pronoun in terms of its quality, quantity, size, color, etc.

Options:
a) Briskly
b) Costly
c) Family
d) Ruthlessly

Explanation:
To identify the adjective out of the given options, we need to understand the function of each word.

1. Briskly: Briskly is an adverb, not an adjective. Adverbs describe verbs, adjectives, or other adverbs. In this case, 'briskly' describes how an action is being performed, not a noun or pronoun.

2. Costly: Costly is an adjective as it describes the quality or characteristics of a noun. It indicates that something has a high cost or price.

3. Family: Family is a noun, not an adjective. It refers to a group of people who are related by blood or marriage.

4. Ruthlessly: Ruthlessly is an adverb, not an adjective. It describes how an action is being performed and modifies a verb, adjective, or another adverb.

Therefore, the adjective out of the given options is Costly. It describes the quality or characteristic of something having a high cost or price.

Summary:
- An adjective is a word that describes or modifies a noun or pronoun.
- The adjective out of the given options is 'Costly' as it describes the quality of something having a high cost or price.