A straight line over a vowel to show it has a long sound is ...

A: breve

B:accent

C:macron

The answer is: C: macron.

Macron refers to the straight line placed over a vowel to give it a longer sound in Latin and Greek languages. For example, ā.

Breve is a short diacritic mark in the shape of the bottom of a half circle. It indicates the vowel is short, such as ă. Likewise, a diacritical mark or accent is used to stress vowels; for example, á.