On Sunday, a significant quantity of ammunition, weapons, and explosives __________ seized from the terrorists.

Explanation

The phrase "a large number of" is treated as a singular unit in this context, despite referring to multiple items like ammunition, weapons, and explosives. Therefore, the singular verb "was" correctly agrees with the subject. Option A "Was" is correct because it matches the singular subject. Option B "We're" is incorrect as it is a contraction of "we are," which is plural. Option C "Has" is incorrect because it is singular but does not fit the phrase "a large number of." Option D "Have" is incorrect as it is plural and does not agree with the singular subject. Option E "Were" is incorrect because it is plural and does not match the singular subject. Hence, the proper verb to complete the sentence is "was," making option A the right choice.