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

Answer: Leftmost derivation.

Why:

• Top-down parsers begin with the start symbol and expand nonterminals. At each step they expand the leftmost nonterminal, so the parser constructs a leftmost derivation of the input.

• The input is scanned from left to right and matched against the terminals produced by these leftmost expansions; LL and recursive-descent parsers follow this approach.

• By contrast, bottom-up parsers (such as LR/shift-reduce) effectively construct a rightmost derivation in reverse, so any option that references a rightmost derivation in reverse refers to bottom-up parsing, not top-down.

Short example:

1. Grammar: S -> a S b | ε

2. Leftmost derivation for input 'a a b b': S => a S b

3. => a a S b b

4. => a a b b (by expanding S -> ε)