What defines the time signature of 3/4?

The time signature of 3/4 is defined specifically by its structure of beats; it indicates that there are three beats in each measure, with each beat represented by a quarter note. This means each measure in a 3/4 time signature is counted as three quarter note beats, making it ideal for styles of music that have a strong emphasis on each of the three beats, like waltzes.

Understanding this, it's clear why other options do not accurately describe the 3/4 time signature. For example, three half notes per bar would suggest a different grouping of beats and wouldn't match the quarter note definition inherent in 3/4. Similarly, while three eighth notes fit within this structure, they do not define the overarching feel and function of the time signature, which is anchored in the quarter note. Three dotted notes would also not apply, as dotted notes inherently alter the duration of the notes and would not conform to the standard beat structure defining 3/4. Hence, the emphasis on quarter notes is what makes option B the correct choice.