Answer: Option B: costs
In this sentence, the correct verb form according to the subject-verb agreement has to be chosen. "Dozen" is a collective noun. One dozen will take a singular verb agreement as an entire dozen (that is 12 items) is considered one as a whole. When a collective noun acts in unison, it takes singular subject-verb agreement. Therefore, "one dozen" here will take the verb singular verb "costs".

Option A is incorrect as "cost" is plural verb agreement.

Option B "costs" is the correct answer.

Option C "costed" is incorrect, as "cost" is an irregular verb whose past form is "cost" as well.

Option D "have costed" is incorrect, as "cost" is an irregular verb whose past participle form is "cost" as well. Here, "have" is incorrect too, as it shows plural verb agreement.