The statement that BEST describes a computer is "It is a mathematical machine."
Computers perform calculations, process data, and execute algorithms, which aligns closely with the definition of a mathematical machine. While they can be considered "thinking" machines in a broad sense due to their ability to perform complex tasks, they do not possess consciousness or independent thinking capabilities like a human.
The other options, "It is a mechanical converter" and "It is a quantum technology," are less accurate in general terms. While modern computers can involve quantum technology, not all computers are quantum computers.