How does a positively charged metal ball become neutral

By gaining equal number of electrons to the charge it carries.

Explanation:

A positively charge metal ball will become if it gains equal number of electrons to the charge it carries.

Generally, a body becomes charged when they lose or gain electrons. The loss or gain of electrons by a body is common place feature. Merely rubbing of substances against each other will cause a loss or gain of electrons.

Electrons are negatively charged particles in an atom

Protons are positively charged particles.

In a neutral atom, the number of protons and electrons must be the same.

A positively charge body will have more number of protons than electrons.

To make a positively charged metal become neutral, we bring it to the vicinity of electrons.