The correct answer is C. Everyone was at the party last weekend.
Explanation
The word "Everyone" is a word that according to the grammar rules of English does not refer to a specific person and is therefore classified as a singular subject. On the other hand, the word "was" is the past tense of the verb TO BE that is used for singular subjects, unlike "were" which is the form of TO BE for plural subjects. Therefore, the correct answer is C. Everyone was at the party last weekend.
