The most appropriate meaning of the underlined idiom "left no stone unturned" is made every possible effort.

The idiom "left no stone unturned" means to do everything possible to find something or to solve a problem. It is often used to praise someone's careful work or to promise a thorough search.

In the sentence "My friends succeeded because they left no stone unturned in their search for an answer to the problem," the speaker is saying that the friends' success was due to their hard work and dedication. They tried everything they could think of to find a solution to the problem.

The other options are not as accurate.

• depended on many people - This option does not accurately reflect the meaning of the idiom. The idiom is about making efforts, not relying on others.
• looked in different places - This option is partially accurate, but it does not capture the full meaning of the idiom. The idiom suggests that the friends looked everywhere possible, not just in a few different places.
• proposed good plans - This option is not accurate at all. The idiom is about making efforts, not proposing plans.