The correct answer is While looking for berries in the forest, we found an old, stone fence.
Explanation:
In general complex sentences contain subordinating conjunctions such as after, before, because, until, etc. that are placed before a dependent clause (incomplete sentence or idea) and connect this clause to two one or more independent clauses or complete sentences. Moreover, the general rule for the punctuation of these clauses is to add a comma between the clauses if the dependent clause is placed first.
This correct punctuation occurs in "While looking for berries in the forest, we found an old, stone fence" because the first section "While looking for berries in the forest" is a dependent clause because it does not express a complete idea and it contains the subordinating conjunction "while" and due to this, a comma is correct between this clause and the independent clause.
