Final answer:
The answer to the fill-in-the-blank question is B) is, following the subject-verb agreement rule where the singular subject 'a virus' takes the singular verb 'is'.
Step-by-step explanation:
The correct answer to the question "A virus in all the company's computers _____ a real threat to security" is B) is. The subject 'a virus' is singular, and therefore, the singular verb 'is' should be used to match the subject in number.
In English grammar, subject-verb agreement is a rule stating that the verb must agree in number with its subject. Singular subjects take singular verbs, and plural subjects take plural verbs.
Therefore, the complete sentence should read: A virus in all the company's computers is a real threat to security.