Hi there!
Since there's no choices attached to this answer, I'll do my best to come up with words that meet your definitions.
1. "a mark placed on, over, or under a letter to indicate pronunciation, stress, and accent"
This is a diacritical mark. In typography, diacritical marks represent a change in pronounciation or the accent.
2. "another word for accent"
Another word for accent could be stress. Accentual tone dictates the way a syllable is stressed.
3. "the system of sounds in a language"
Linguistically, the system of sounds in another language refers to the classification of sounds, and within systems, different things are classified. Therefore, the word you're looking for is phonetics.
