Question

atoms of two different elements must have different
electrical charges.
numbers of neutrons.
atomic numbers.
energy levels.

Explanation:

To solve this, we recall the definition of an element: an element is defined by its atomic number (number of protons in the nucleus). Atoms of different elements must have different atomic numbers (since atomic number determines the element). Electrical charges can vary (e.g., ions of the same element), number of neutrons can vary (isotopes of same element), and energy levels depend on electron configuration which can be similar for different elements.

Answer:

C. atomic numbers