Let $G_1, G_2$ be Context Free Grammars (CFGs) and $R$ be a regular expression. For a grammar $G$, let $L(G)$ denote the language generated by $G$. Which ONE among the following questions is decidable?

Let us check the decidability of the given decision problems for Context-Free Languages (CFLs):

1. Emptiness of a CFG ($L(G_1) = \emptyset$): This is decidable. It can be solved by determining whether any terminal string can be derived from the start variable (by iteratively marking variables that generate terminal strings).

2. Equivalence of two CFGs ($L(G_1) = L(G_2)$): This is undecidable.

3. Intersection-emptiness/Disjointness ($L(G_1) \cap L(G_2) = \emptyset$): This is undecidable.

4. Equivalence of a CFG with a Regular Expression ($L(G_1) = L(R)$): This is undecidable (since checking universality $L(G_1) = \Sigma^*$ is a special case where $R = \Sigma^*$, which is undecidable).

Therefore, only Is $L(G_1) = \emptyset$? is decidable, which corresponds to option D.