What is CPython?

It's the default, reference implementation of the $C$ language, written in Python

It's the default, reference implementation of Python, written in the C language

It's a programming language that is a superset of Python, designed to produce C$-$like performance with code written in Python

It's a programming language that is a superset of the $C$ language, designed to produce Python$-$like performance with code written in C