Which describes the notation tt for the trait of being tall?

a. homozygous genotype

b. heterozygous genotype

c. homozygous phenotype

d. heterozygous phenotype

The answer is B. Heterozygous genotype. 

The notation Tt shows a heterozygous genotype. A genotype shows the set of genes that it carries or was inherited. The capital letter represents a dominant trait and a lower case letter represents a recessive trait. 

There are two ways to describe them; homozygous and heterozygous. 

Heterozygous genotype has one dominant trait and one recessive trait. It looks just like what is given in your problem. Homozygous, on the other hand, has either a pair of the dominant trait or a pair of the recessive trait. 

This is how they look like:
heterozygous: Tt
Homozygous: tt

Now phenotype is different. Phenotype is the trait that is physically expressed. The trait that will be expressed will depend on the genotype. 
Genotype                     Phenotype
     TT =                      homozygous tall
      tt =                              short
     Tt =                       heterozygous tall 

Take note that the dominant trait is the trait that will physically manifest or physically be expressed.