Use a after an independent clause that introduces a clause, a phrase, or even a single word that explains the original clause

Colon.

Explanation:

As per the rules, a colon(:) is usually employed for several purposes like introducing a list or series of items, initiate an appositive or quotation the sentence.

In order to link two independent clauses also a colon(:) is employed if the later clause functions to explain, illustrate, or modify the prior clause. The first clause creates a supposition that is accomplished by the introduction of the later clause. For example:

"I have a list of works to be done: first, I need to pack my bags for tomorrow; then, I require to make sure that everything is placed appropriately."

yes yes it does !

i think its d, but grammarly says none

explanation: