The correct answer is: B. are building
The verb “build” is a regular verb, which means that the past tense and past participle are formed by adding -ed to the present tense form. However, when “build” is used in the present continuous tense, the verb “be” is used before it, and the -ing form is used.
In this case, the sentence is about the present, so the present continuous tense is used. The subject of the sentence is “houses”, which is plural, so the verb “are” is used.
The other options are incorrect because they are not in the present continuous tense. Option A, “builds”, is in the simple present tense, which is used to talk about general truths or habits. Option C, “built”, is in the simple past tense, which is used to talk about things that happened in the past. Option D, “are built”, is in the passive voice, which is used to focus on the action rather than the person or thing doing the action.
In conclusion, the correct answer is “are building” because it is the only option that is in the present continuous tense.