Questions: A trait is a discrete with a known quality.

The answer is the fourth one: characteristic.

Explanation for each option:

1. Blood cell: This is incorrect. A blood cell is a type of cell found in the blood, not a trait or a discrete unit with a known quality.

2. Allele: This is incorrect. An allele is a variant form of a gene, not a trait itself. Alleles contribute to the expression of traits, but they are not the traits.

3. Gene: This is incorrect. A gene is a segment of DNA that contains the instructions for the development of a trait, but it is not the trait itself.

4. Characteristic: This is correct. A characteristic is a trait or feature of an organism that can be observed and described. It is a discrete unit with a known quality, such as eye color or height.

In summary, a trait is best described as a characteristic, which is a discrete unit with a known quality.