Compiler front end (PL/I and general languages): during compilation, the stage called Syntax analysis (parsing) is primarily responsible for which activity?

Introduction / Context:
Compiler front ends separate concerns into lexical analysis, syntax analysis, and semantic analysis. Parsing (syntax analysis) ensures that token sequences form valid statements according to the grammar of the language. This question focuses on the role of parsing specifically for PL/I, but the ideas are language-agnostic.

Given Data / Assumptions:

• Input to the parser is a stream of tokens (uniform symbols) from lexical analysis.
• The grammar is context-free or extended, enabling productions and reductions.
• We distinguish parsing from lexing and from macro processing or optimization.

Concept / Approach:
Parsing recognizes syntactic constructs (e.g., expressions, statements, declarations) by applying grammar productions. Bottom-up parsers perform reductions from tokens to nonterminals until a start symbol is formed; top-down parsers derive structures from the start symbol. Either way, the parser checks structural correctness and builds parse trees or abstract syntax trees (ASTs).

Step-by-Step Solution:
1) Start with tokens from the lexer (identifiers, literals, operators).2) Apply grammar rules to group tokens into phrases like terms, expressions, and statements.3) Use reductions or derivations to confirm structures and build an AST.4) Forward the AST to semantic analysis and later stages (IR generation).

Verification / Alternative check:
Standard texts describe LR/LALR parsers performing shift-reduce actions; the reductions correspond to recognizing syntactic constructs. This matches option A.

Why Other Options Are Wrong:

• Option B describes lexical analysis (tokenization), not parsing.
• Option C is not a recognized compiler phase.
• Option D concerns macro processing/optimization, which are separate from parsing.

Common Pitfalls:
Mixing tokenization with parsing, or believing macros are part of syntactic validation. Keep phases modular to simplify compiler correctness and maintenance.

Final Answer:
recognition of basic syntactic constructs through reductions.