Plural of thief . . . . . . . . thieves. A. was B. were C. are D. is

The correct answer is thieves.

The plural of thief is thieves. This is because the word thief is a regular noun, which means that the plural form is formed by adding -s or -es to the end of the singular form.

The other options are incorrect because they are not the plural form of the word thief. The word was is the past tense form of the verb to be, the word were is the past tense form of the verb to be for plural subjects, the word are is the present tense form of the verb to be for plural subjects, and the word is is the present tense form of the verb to be for singular subjects.