Answer: (iii) Leftmost derivation

Explanation

A top-down parser starts from the start symbol (root) and tries to derive the input string by expanding nonterminals in leftmost order — i.e. it always expands the leftmost nonterminal first.

• Leftmost derivation: each step replaces the leftmost nonterminal — this is exactly what a top-down (recursive-descent / predictive / LL) parser constructs.
• Rightmost derivation: each step replaces the rightmost nonterminal — not what a top-down parser does.
• Rightmost derivation in reverse / handle pruning: that is the basis of bottom-up parsing (LR parsers), not top-down.

Therefore a top-down parser generates a

$$\boxed{\text{(iii) Leftmost derivation}}$$