The correct answer is option 4: a theoretical computing machine to act as an ideal model for mathematical calculation.

A Turing machine is a theoretical device that was proposed by Alan Turing in 1936. It is an abstract concept used in the field of computer science and mathematics to study the limits of computation.

A Turing machine consists of a tape with an infinite length divided into cells, a head that can read and write symbols on the tape, and a set of defined rules that dictate the machine`s behavior. The machine starts in an initial state and follows these rules to perform calculations.

This theoretical model is used to understand the capabilities and limitations of algorithms and computation. It helps computer scientists and mathematicians analyze the complexity of problems and design efficient algorithms to solve them.

In summary, a Turing machine is not used to identify explosives, inform about forest fires, or find out the gravitational constant. Its purpose is to serve as a theoretical computing machine for mathematical calculations and research.