________predicate calculus allows quantified variables to refer to objects in

Correct answer: First-order predicate calculus.

Explanation: In first-order logic, quantifiers (for all, there exists) bind variables that range over individual objects in the domain of discourse. Predicates and functions are not objects of quantification; they are used to describe or map those objects.

• Zero-order (propositional) logic: no quantifiers over objects; formulas are built from propositional variables and logical connectives.

• First-order logic: quantifies over individual objects. Example: ∀x (Human(x) → Mortal(x)) quantifies the variable x, which ranges over people.

• Second-order logic: allows quantification over predicates, relations, or sets (for example, quantifying over P), so it is more expressive but different from the description in the question.

• Higher-order logics: permit quantification over functions and predicates of higher types; these also do not match the description that quantifiers only range over objects.

Tip: Remember that the phrase in the question pinpoints where quantifiers range: if they range over individual objects, it is first-order logic; if they can range over predicates or functions, it is second-order or higher-order logic.