What law best explains why an organism with tt will show only the dominant phenotype

This is the law of dominance in genetics. The dominant allele will mask the effects of the recessive allele and therefore will be the visible trait of the phenotype. Most often, the dominant allele codes for functional proteins, while the recessive does not code for functional proteins. Dominance in genetics is significant in Mendelian inheritance.  

The law of dominance explains why an organism with Tt will show only the dominant phenotype.

Explanation:

The law of dominance devised by Gregor Mendel in the 1860s. Mendel who is fondly called as the “Father of Genetics” conducted experiments on pea plants for about 7 years and has formulated a hand full of laws namely the law of dominance, law of segregation and law of independent assortment.

All these laws collectively express the transfer of characters from one generation to another. Later these laws were accepted universally with a few exceptions. They gave rise to new terminologies such as a dominant trait and a recessive trait.