Which molecule could he make that consists of long chains of red and black colored balls

He could make a macromolecule which is made up of carbon and hydrogen only.

Explanation:

An atom a fundamental piece of matter that constitutes a chemical element.

A molecule is a group of atoms bonded together. It represents the smallest fundamental unit of a chemical compound that can participate in a chemical reaction.

The red balls denote the hydrogen and black balls denote the carbon. He could make a macromolecule which is made up of carbon and hydrogen only.