Introduction:
Turing machines are theoretical devices that were introduced by Alan Turing in 1936. They are used to understand the concept of computability and the limits of what can be computed. A Turing machine consists of an infinite tape divided into cells, a read/write head that can move along the tape, and a finite control unit that determines the machine's behavior.

Equivalent Models:
There are various equivalent models to a Turing machine, which means that these models have the same computational power and can solve the same problems. The options given in the question are:

a) Multi Tape Turing Machine:
In a multi-tape Turing machine, there are multiple tapes instead of a single tape. Each tape has its own read/write head, and the machine's behavior is determined by the combination of the states of all the heads. However, this model is equivalent to a Turing machine because it can simulate a single-tape Turing machine by using one tape to represent the input and output of the computation.

b) Multi Track Turing Machine:
In a multi-track Turing machine, there are multiple tracks on a single tape. Each track can store a symbol independently, and the heads can move in different directions. This model is also equivalent to a Turing machine because it can simulate a single-track Turing machine by using one track to represent the input and output.

c) Register Machine:
A register machine is a type of computer that consists of a set of registers, where each register can store an integer value. It also has a program counter that points to the current instruction, and a set of instructions that operate on the registers. Although a register machine is a different model than a Turing machine, it is still equivalent in terms of computational power. This means that any problem that can be solved by a Turing machine can also be solved by a register machine.

Conclusion:
All the models mentioned in the options (multi-tape Turing machine, multi-track Turing machine, and register machine) are equivalent to a Turing machine. This means that they have the same computational power and can solve the same problems. Therefore, the correct answer is option 'D' - All of the mentioned.