Assertion-Reason: IRT
Assertion (A): IRT is commonly used in computerized psychological testing to estimate ability. Reason (R): In IRT, four parameters can be estimated.

Correct option is B

Item Response Theory (IRT) is indeed widely used in computerized adaptive testing (CAT) to estimate examinee ability (A is true). It's also true that some IRT models (e.g., the 4-parameter logistic model) estimate up to four parameters — discrimination, difficulty, guessing, and an upper asymptote (R is true). However, the number of parameters estimable in a model does not explain why IRT is used in computerized testing; the real reason is IRT's item-level, ability-invariant measurement properties that allow adaptive item selection. Hence R does not correctly explain A.
Information Booster:
1. Common IRT models: 1PL (Rasch), 2PL, 3PL, and 4PL.
2. CAT selects items dynamically based on the examinee's estimated ability level.
3. IRT parameters: discrimination (a), difficulty (b), guessing (c), and upper asymptote (d).
4. IRT is favored over Classical Test Theory for adaptive and equating purposes.