Which of the following statements is true regarding Turing machines?

a$.$ Every Turing machine can be converted to an equivalent deterministic finite automaton.

b$.$ The Halting Problem is solvable for all Turing machines.

c$.$ Non$-$deterministic Turing machines are more powerful than deterministic Turing machines.

d$.$ The set of languages recognized by Turing machines is exactly the set of recursively enumerable languages.