Nested Stack Automaton allows stacked values to be sub-stacks rather than just finite symbols.

Nested Stack Automaton:
A Nested Stack Automaton (NSA) is an extension of a Push Down Automaton (PDA) in which the stack alphabet consists of stacks instead of just symbols. It is a theoretical model of computation that can accept or reject a given input string based on a set of rules.

Push Down Automaton:
A Push Down Automaton (PDA) is a finite state machine with an added stack that allows it to perform stack operations such as push, pop, and peek. The stack is used to store symbols from the input string while the finite state machine transitions between states based on the current input symbol and the top of the stack.

Turing Machine:
A Turing Machine is a theoretical computing device that can manipulate an infinite tape divided into cells. It consists of a tape head that can read and write symbols on the tape, a set of states, and a set of transition rules that determine how the machine moves and changes its state based on the current symbol and state.

Explanation:
The Push Down Automaton (PDA) and Turing Machine both use a finite alphabet of symbols to represent the stack. In a PDA, the stack can only store symbols from the input string, while in a Turing Machine, the tape can store any symbol from a finite alphabet.

However, in a Nested Stack Automaton (NSA), the stack alphabet consists of stacks instead of just symbols. This means that each stack can have its own set of symbols, allowing for nested sub-stacks within the main stack. The NSA can perform stack operations on these sub-stacks, such as push, pop, and peek, similar to a PDA.

The ability to have nested sub-stacks in an NSA makes it more powerful than a PDA or Turing Machine, as it can handle more complex computations and languages. It provides a way to represent hierarchical structures and can be used to parse and process nested expressions or languages.

Therefore, the correct answer is option 'C' - Nested Stack Automaton, as it allows stacked values to be sub-stacks rather than just finite symbols.