A test cross is carried out:

Options

A. Determine the genotype of a plant of `F_(2)`
B. Predict whether two traits are linked
C. Assess the number of alleles of a gene
D. Determine whether two species or varieties will breed successfully

Answer

### Step-by-Step Solution:

1. **Understanding Test Cross**: A test cross is a genetic cross between an individual showing a dominant phenotype (but unknown genotype) and a homozygous recessive individual. This helps determine the genotype of the dominant phenotype individual.

2. **Purpose of Test Cross**: The primary purpose of a test cross is to identify whether the dominant phenotype individual is homozygous dominant (AA) or heterozygous (Aa) for the trait in question.

3. **Example of Traits**: Consider a trait such as plant height, where tall (T) is dominant over dwarf (t). If we have a tall plant, we cannot determine if it is TT (homozygous dominant) or Tt (heterozygous) just by observing its height.

4. **Performing the Test Cross**: To perform a test cross, we cross the tall plant (unknown genotype) with a dwarf plant (tt). The offspring (F1 generation) will help us determine the genotype of the tall parent.

5. **Analyzing the Results**:
- If all offspring are tall, the tall parent must be homozygous dominant (TT).
- If the offspring are a mix of tall and dwarf plants, the tall parent must be heterozygous (Tt).

6. **Conclusion**: Thus, a test cross is specifically used to determine the genotype of a plant, particularly in the context of F2 generation analysis.

### Final Answer:
A test cross is carried out to determine the genotype of a plant of F2.