Today, we used the boat <u>A) that our grandfather built </u>instead of the other boat.
The chosen sentence is an<u> essential adjective clause</u>, meaning that it acts as an adjective, that is to say, it describes or gives characteristics to the noun "boat", and it's essential since the reader needs the information of the clause to know what boat the speaker is referring to.
Option A is the correct answer because the grammar rule holds that essential clauses do not require commas. Unlike nonessential ones, who does require commas.
