What was Guido d'Arezzo's contribution to musical notation?

A. Hexachords

B. Staff Lines

C. Rhythm

D. Articulation Marks

E. Neumes

The answer is : staff lines. He devised an arrangement of lines and spaces that developed into the musical staff. Two of Guido D'Arezzo's primary contributions to music history : 1. developed a 4-line staff circa. 1025 2. wrote a text on music ca. 1026 called "Micrologus"

staff lines

Explanation:

its the answer for apex