GATE CS 1987 Q1-xii — Parsing (LL, LR)

By definition, a context-free grammar (CFG) $G$ is ambiguous if there exists at least one string $w \in L(G)$ that has more than one distinct parse tree (or equivalently, more than one leftmost derivation or rightmost derivation).

Let's evaluate the given options:

• Option A: Useless non-terminals are symbols that cannot derive any terminal string or cannot be reached from the start symbol. Their presence does not determine whether a grammar is ambiguous.
• Option B: This directly matches the standard definition of ambiguity.
• Option C: Having adjacent non-terminals on the right-hand side (e.g., $A \rightarrow BC$) is a standard feature of many unambiguous grammars, such as those in Chomsky Normal Form (CNF). It does not imply ambiguity.

Therefore, the correct option is B.