Words which one recognizes when one hears or sees them are __________.

Vocabulary knowledge is classified along two independent axes. The first is reception versus production: receptive vocabulary is the set of words a person can recognize and understand when they hear or read them, while productive vocabulary is the set of words a person can actively recall and use correctly in their own speech or writing. The second is explicit versus implicit learning, which describes HOW a word was learned, through direct, conscious instruction versus incidental exposure, and is independent of whether the word can be recognized or produced.

The blank in the stem describes words that a person recognizes when one hears or sees them. This is precisely the reception side of the reception/production axis, understanding a word on encountering it, not necessarily being able to produce it unprompted. So the term being asked for is the one that names recognition-based word knowledge: receptive vocabulary.

Checking against each option:

• Productive vocabulary names words a person can actively recall and use in speech or writing, a production capability, not mere recognition.

• Receptive vocabulary names words a person recognizes and understands on hearing or reading them, matching the stem's description exactly.

• Explicit vocabulary describes words learned through direct, conscious instruction, this classifies the learning method, not a recognition-versus-production capability.

• Implicit vocabulary describes words learned incidentally through exposure, without deliberate instruction, again a learning-method label, not a recognition-versus-production capability.

Since the stem asks specifically about recognition on hearing or seeing a word, the answer is receptive vocabulary.