What you already know:
C on this diagram is a proton and B points to the electron. A note the nucleus that contains the protons and the center of orbit to the electrons. D notes the electron cloud, radius, or the bounds to which the electron obitals exist.
If you count the number of protons this atom has it adds up to 3, which, therefore comfirms that this molecule is Li. The number of protons is specific to the atom. The question states that this element has a mass number of 7, this is because the mass number is the sum of both protons and electrons.