Final answer:
In the sentence 'In front of both houses (are, is) a yard sale sign,' the correct verb is 'is' because the subject 'a yard sale sign' is singular.
Step-by-step explanation:
The subject in the sentence 'In front of both houses (are, is) a yard sale sign' is 'a yard sale sign.' Therefore, the correct verb to use is 'is,' because 'a yard sale sign' is singular. Despite the presence of two houses, the sentence's subject is the singular 'yard sale sign,' not the houses themselves. It's akin to saying there is a sign that happens to be in front of both houses, which doesn't change the singularity of the subject.