In a typical test cross:

The correct answer is Option (4) → An organism (pea plant) showing a dominant phenotype is crossed with a recessive parent to determine its genotype.

Option (1) An organism (pea plant) showing dominant phenotype is crossed with the dominant parent. — Incorrect. A cross with the dominant parent (TT) gives only dominant offspring, whether the test plant is TT or Tt. So it cannot show the hidden recessive allele.

Option (2) An organism (pea plant) showing recessive phenotype is crossed with dominant parent to determine its phenotype. — Incorrect. Two things are wrong. A recessive-looking plant must be tt, so its genotype is already known. And its phenotype is seen directly, so no cross is needed to find it.

Option (3) An organism (pea plant) showing dominant phenotype is self-crossed to determine its genotype. — Incorrect. Self-crossing a plant is selfing, the way Mendel raised the F₂ generation. A test cross always uses a recessive parent, not the plant itself.

Option (4) An organism (pea plant) showing a dominant phenotype is crossed with a recessive parent to determine its genotype. — Correct. A tall plant may be TT or Tt. Crossed with a dwarf (tt) plant, a TT plant gives all tall offspring, while a Tt plant gives tall and dwarf in a 1:1 ratio. The offspring thus show the genotype.

Note: A similar question was also asked in CUET (UG) 2025 on 30-05-2025 (Slot 1).

NCERT Extract:
Inheritance of One Gene – …