Which type of traits are specifically described as being must-counted?

Meristic traits are specifically characterized as must-counted traits because they are measurable and can only take on whole number values. These traits are often based on a count of discrete items or occurrences, such as the number of seeds in a pod or the number of petals on a flower. Each trait can be counted and falls into a distinct category, which makes it significant in genetic studies and analyses.

In contrast, qualitative traits are more subjective and typically describe qualities or characteristics that can't be reduced to a count, like flower color. Polygenic traits involve the combined effect of multiple genes influencing a single trait and usually exhibit continuous variation, making them less suited for strict counts. Simple traits, while they may have distinctive phenotypes, often focus on single-gene influences without the counting aspect that defines meristic traits. Thus, meristic traits stand out as those with specific counts that can be directly measured, validating the selection of this option.