To solve this question, the user needs to have knowledge of subject-verb agreement. In this sentence, the subject is "houses built before World War II," which is plural. Therefore, the correct verb form should also be plural.

Now let's go through each option and explain why it is right or wrong:

A) "have" - This option is correct because "have" is the correct plural form of the verb "to have". Therefore, "houses built before World War II have lower costs than more modern homes do" is grammatically correct.

B) "has" - This option is incorrect because "has" is the singular form of the verb "to have". It does not agree with the plural subject "houses built before World War II".

C) "None of the above" - This option is incorrect because option A is the correct answer.

D) "Both a and b" - This option is incorrect because option A is correct, and option B is incorrect.

Therefore, the answer is: A) have.