An atom containing three protons and three neutrons in its nucleus has a (an)?

The correct answer is the atomic number of the atom, determined by the number of protons in its nucleus. In this case, the atom has three protons, which directly defines its atomic number as 3. The atomic number is fundamental in identifying the element and determines its position in the periodic table, influencing its chemical properties and behavior.

The other options provide different definitions. The atomic weight refers to the weighted average of the masses of the isotopes of an element, and it's typically not an integer. The mass number, which is the sum of protons and neutrons in an atom's nucleus, would be 6 for this atom (3 protons + 3 neutrons). Atomic mass typically refers to the mass of an individual atom, commonly expressed in atomic mass units (amu), which also involves isotopic composition and is not applicable here.