Active vocabulary consists of words which are _____.

The correct option is used frequently.

Understanding Active Vocabulary:
Vocabulary in language acquisition is broadly divided into two main categories: active vocabulary and passive vocabulary.

1. Active Vocabulary: Refers to the set of words that a language learner or speaker understands well and uses regularly and confidently in daily communication, both in speech and in writing. Because these words are recalled easily when expressing thoughts, they are used frequently.

2. Passive Vocabulary: Refers to words that a person recognizes and understands when reading or listening, but does not actively use in their own speech or writing.

Therefore, active vocabulary consists of words that are used frequently in active expression.