You can physically see the
of a trait, but not

options are
genotypes
phenotypes

A) Phenotype

B) Genotype

Explanation:

Phenotypes are the visible physical features while the genotype is set of alleles (having similar of different traits /characteristics). Alleles are microscopic genetic material which are not visible through naked eyes.  

For example –  

Let the allele for short height be represented by “t” and allele for tall height be represented by “T” and  “t” is recessive to “T”

The genotype will be set of two alleles (which may be same or different).  

Genotype “TT” represents tall people

Genotype “tt” represents short people and Genotype “Tt” also represents tall people  

The height (phenotype) is visible physically but the genotype of height is not visible.

Like wise phenotype of all other characteristics such as eye color, skin color, hair color, any genetic disease etc, is visible physically thought its genotype remains hidden.