Which of the following problems are undecidable?

GATE · 1989 · CS · Question 3 subparts

1. A.

   Membership problem in context-free languages.

2. B.

   Whether a given context-free language is regular.

3. C.

   Whether a finite state automation halts on all inputs.

4. D.

   Membership problem for type $0$ languages.

Attempted by 2 students.