Answer:
c) Had found.
Step-by-step explanation:
Considering the structure of the sentence, it is easy to conclude that it is in the past tense. Thus, the use of the word "turned" in the first sentence is justified.
Now, the second sentence tells what had happened. So, we will use the past perfect tense in it. This means we will use "had+3rd form of the verb" in it. That makes the sentence as "they had found it in the street". This use of the perfect tense makes it clear that the second sentence happened before the first sentence and is a clarification of what had happened.
Thus, the correct answer is option (c) had found.