If I know the number of protons in a neutral atom, what is another
subatomic particle I know?

Number of electrons

Explanation:

An atom is made up of three fundamental subatomic particles called protons, neutrons and electrons.

Protons are the positively charged particles in an atom.

Neutrons do not carry any charges

Electrons are negatively charged.

 Both protons and neutrons occupy the tiny nucleus. The extra-nuclear space is filled with electrons.

the correct option is c.

isotopes refers to different forms of the same element with the same atomic number but different mass number as a result of difference in their neutron numbers. isotopes have different relative atomic mass but share the same chemical properties.