1. what is the subordinate clause in this sentence? as i was walking, i stumbled upon a beautiful seashell. a. as i was walking b. i stumbled upon c. a beautiful seashell

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!