Many young men were ___ at street corners for the coffee bar to open.

The correct answer is Hanging about. This phrasal verb means to spend time idly in a place, often waiting or doing nothing in particular. This perfectly fits the context of young men waiting for a coffee bar to open.

• Hanging back means to hesitate or to remain behind.
• Hanging on can mean to hold tightly, to persist, or to wait for a short period, but it doesn't convey the idleness implied here.
• Hanging in means to persevere or to continue to try hard.

None of the other options accurately describe the act of loitering or waiting aimlessly in a public place.