Which of the following is/are undecidable? 1. $G$ is a CFG. Is $L(G) = \Phi$? 2. $G$ is a CFG. Is $L(G) = \Sigma^*$? 3. $M$ is a Turing machine. Is $L(M)$ regular? 4. $A$ is a DFA and $N$ is an NFA. Is $L(A) = L(N)$?

Let us evaluate the decidability of each problem:

1. Emptiness problem for CFGs ($L(G) = \emptyset$): Decidable. We can test if any terminal string can be derived from the start symbol using the generating/reachable variables algorithm.

2. **Universality problem for CFGs ($L(G) = \Sigma^*$): Undecidable**. Testing if a context-free grammar generates all strings over its alphabet is a well-known undecidable problem (can be reduced from the Post Correspondence Problem or TM halting problem).

3. Regularity of a Turing Machine language ($L(M)$ is regular): Undecidable. By Rice's Theorem, any non-trivial semantic property of the language accepted by a Turing machine is undecidable.

4. Equivalence of a DFA and an NFA ($L(A) = L(N)$): Decidable. We can convert the NFA $N$ to an equivalent DFA $D_N$, construct the symmetric difference DFA $M_{diff} = (A \cap \overline{D_N}) \cup (\overline{A} \cap D_N)$, and test if $L(M_{diff}) = \emptyset$.

Therefore, only statements 2 and 3 are undecidable, which corresponds to option (D).