Top-down Parser:
A top-down parser is a type of parsing algorithm that starts with the root of the parse tree and works its way down to the leaves. It begins with the start symbol of the grammar and repeatedly applies production rules to generate the input string. Top-down parsers are also known as predictive parsers because they predict which production rule to apply based on the next input symbol.

Recursive Descent Parser:
A recursive descent parser is a type of top-down parser that uses a set of recursive procedures to parse the input. Each non-terminal in the grammar corresponds to a procedure, and the parser calls these procedures recursively to generate the parse tree. The recursive descent parser starts at the root of the parse tree and recursively applies the production rules until it reaches the leaves.

Operator Precedence Parser:
An operator precedence parser is a type of bottom-up parser that uses a stack to parse the input based on the precedence of operators. It scans the input from left to right and reduces the input to the leftmost non-terminal symbol. Operator precedence parsers are commonly used to parse arithmetic expressions.

LR(k) Parser:
An LR(k) parser is a type of bottom-up parser that uses a parsing table to parse the input based on a look-ahead of k symbols. LR(k) parsers are powerful and can handle a wide range of context-free grammars. They are commonly used in practice due to their efficiency and generality.

LALR(k) Parser:
An LALR(k) parser is a variant of the LR(k) parser that uses a less powerful but more compact parsing table. LALR(k) parsers are commonly used in practice as they strike a balance between power and efficiency.

Explanation:
Among the given options, the recursive descent parser is the only one that is a top-down parser. It starts at the root of the parse tree and uses recursive procedures to generate the input string. The other options, operator precedence parser, LR(k) parser, and LALR(k) parser, are all bottom-up parsers that start with the input and build the parse tree from the leaves to the root.

Therefore, the correct answer is option 'A' - Recursive descent parser.