Answer:
As he walked along the road, he came upon a deserted house in the middle of the open fields.
Step-by-step explanation:
As given in the question, "as he walked along the road" is not a complete sentence. Rather, it requires an independent clause to be a complete sentence.
An independent clause is one that can stand alone as a complete sentence while a dependent clause requires an independent clause to be a full sentence.
So, to the clause "as he walked along the road", we add "he came upon a deserted house in the middle of the open fields" to make the whole sentence.
Thus, the completed sentence becomes -
As he walked along the road, he came upon a deserted house in the middle of the open fields.