$L = \{ \langle M \rangle \mid \text{there is at least one input string on which the \(M\) does not halt} \}$
Here, for a Turing machine $M$, the notation $\langle M \rangle$ denotes an encoding, over some alphabet, of the code of the Turing machine. To which of the following language classes does $L$ belong?
- Regular.
- Context-free but not Regular.
- Recursive but not Context-free.
- Recursively enumerable but not recursive.
- Not recursively enumerable.