The symbols used in the earliest form of european musical notation were called:

Neumes  

Explanation:

    Neumes are the basic elements of the music notation system before the invention of five-line stave notation. Musical notation first came into being with the function of aiding the memory of the singer and only later became more and more accurate. There were also some changes to neumes, such as subpunctis, resupinus, and flexus.

your answer would be d.)