The climber was unable to scale the precipitous cliff side.
The man didn't realize how preciptious the cliff was until after begining to climb it.
When it has a subject, verb, punctuation, capitalization, a predicate, and a complete thought. A sentence is a sentence when it is handed down by a judge (or a magistrate).
This is one of a series of questions copied from Penn-Foster's "Practical English" quiz.There is no answer here.
The subject and predicate carry most of the content of a sentence.