When you have a writing assignment, which of the following activities would be the BEST way to start the writing process?

Prewriting, drafting, revising, and editing are at least four separate stages in the writing process. The first and most critical step is to determine why you're writing (to advise, clarify, or for some other reason) and for whom you're writing. Writing is a multi-step process that includes brainstorming (pre-writing), organizing, writing, revising, and editing. These various elements, however, do not exist in isolation; they are not distinct phases. Ses various aspects are continually examined by successful authors.

A subordinate clause is a clause that cannot stand alone as a complete sentence.

Subordinate clauses usually begin with subordinating conjunctions which link dependent clauses to independent clauses, such as for, as, since, therefore, hence, consequently, due to, though, provided that, because, unless, once, while, when, whenever, where, wherever, before, and after.

They can also begin with relative pronouns such as that, which, who, whom, whichever, whoever, whomever, and whose.

To identify a subordinate clause, there must always have a comma after it. 

When the main clause starts the sentence, there is no comma to separate it from the dependent clause.

Now, let's look over the sentence:

"As I was walking, I stumbled upon a beautiful seashell."

If we use this info, we can deduce that the answer is (A. As I was walking).

I hope this helped!