Which of the following derivations does a top-down parser use while parsing an…

A top-down parser constructs a parse tree starting from the root (the start symbol) and works its way down to the leaves. At each step, it selects a non-terminal and replaces it with one of its production rules. Crucially, to match the input string scanned from left to right, the parser must always expand the leftmost non-terminal in the current sentential form. This process generates a sequence of strings where each step replaces the leftmost non-terminal, which is precisely the definition of a leftmost derivation. Therefore, option A is correct.

Option B describes reverse top-down parsing or bottom-up techniques incorrectly. Option C and D refer to rightmost derivations, which are characteristic of bottom-up parsers (like LR parsers) that reduce the input string from left to right, effectively building a rightmost derivation in reverse. Since top-down parsing proceeds by predicting and expanding the leftmost symbols first, it strictly follows a leftmost derivation path.