In parsing theory, what concept represents the set of possible parser states at a given point, including the current input position?

• A. Closure
• B. State Space
• C. Transition System
• D. Parse Tree

Answer: (A)

The closure operation defines the set of parser states (items) active at a given input position by expanding all possible next steps a parser could take. In parsing formalisms like LR and Earley, a state is represented as items with a dot indicating how much of a production has been seen, and the current input position is part of that item. By applying the closure, you include every production for any nonterminal that appears immediately to the right of the dot, at the same input position, so the state reflects all possible continuations from that point. This yields the complete picture of where the parser could be and what it could derive next, which is exactly what’s meant by the set of possible parser states at that point. The other terms are broader or refer to different concepts: a state space is the whole collection of states the machine might ever reach, a transition system is the model of moves between states, and a parse tree is a single, fully derived structure rather than the current set of active possibilities.