Answer: Option C, String quartet.
Explanation:
Let's analyze the options one by one:
In the divertimento, you may see more than four instruments, so this option can be discarded.
The serenade was invented in the eighteenth century, but you also may see wind instruments here, so this option can be discarded.
The string quartet sounds like a good option, it is usually confirmed by two violins, a viola and a cello (all instruments of string), this type of composition started in the eighteenth century, so this is the right option.
