Which one of the following is a top-down parser?

Answer: Recursive descent parser.

Explanation: Why this is correct and why the others are not.

• Top-down parsing starts from the grammar's start symbol and expands nonterminals to generate the input (producing leftmost derivations).

• Recursive descent parser: a direct implementation of top-down parsing using mutually recursive procedures for nonterminals; predictive versions use lookahead to select productions without backtracking.

• Operator precedence parser: a bottom-up shift-reduce approach that reduces tokens according to operator precedence, so it is not a top-down parser.

• LR(k) and LALR(k) parsers: bottom-up (shift-reduce) parsers that build a rightmost derivation in reverse; they do not perform top-down expansion.