In this sentence, we are talking about the failure rate of new businesses. The subject "one of every two new businesses" is singular, so we need a singular verb to agree with it.

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

A. fail: This option is incorrect because "fail" is a plural verb and does not agree with the subject "one of every two new businesses."

B. fails: This option is correct. "Fails" is a singular verb that agrees with the subject "one of every two new businesses."

C. should fail: This option is incorrect because it suggests an obligation or recommendation for the businesses to fail, which is not the intended meaning of the sentence.

D. may have failed: This option is incorrect because it implies uncertainty about the failure of the businesses, whereas the sentence presents a general statement about the failure rate.

Therefore, the correct answer is: B. fails.