Your answer is D. The sum of the protons and Neutrons in the atom.
Protons and neutrons are the larger particles, and are found in the nucleus, which is the core of the atom.
Atomic mass is typically calculated by adding the number of protons and neutrons together, ignoring the electrons because of their small size.
For more you may visit- http://study.com/academy/lesson/what-is-atomic-mass-definition-examples-quiz.html
Hope this Helps!!