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).
Explanation: This means that there is always something positive, and something negative in a situation. There is always a silver lining on every grey cloud. A situation is bad if you look at it negatively, but it will be better once you look at it from a positive perspective.