The verb which correctly completes the sentence is were.
A verb is one of the parts of speech in English language. Verbs can be defined as any word that describes an action, occurrence, or state of existence. Some examples of verb are dance, sing, walk, play, run, sit, clap, cheer, etc.
In English language, a subject can be defined as the person, thing, idea or place that performs an action.
On a related note, a subject-verb agreement is a situation in which there exist an agreement between a subject and a verb in terms of number.
This ultimately implies that, if a subject is singular, its verb should also be singular while if the subject is plural, the verb must be plural.
In this context, the most appropriate verb to use in completing the sentence is "were" because a subject (crowd) is plural.
Note: If the subject is singular, you would use is.
In conclusion, the complete sentence would be written as; "The crowd were clapping and cheering."