The coach explained the rules that the officials revised in 2010.
The essential adjective clause is one that is essential to the sentence. It tells us information that we need to know. In this sentence, we need to know which rules the coach explained. Essential adjective clauses can not be removed from the sentence because you would not have all of the information needed. Essential clauses do not use commas. With this in mind, let's look at the other options. They all contain commas to set off the adjective clauses which tells us they are not essential.
