If you are working with two tall pea plants and know that one is tt, how could you determine the genotype of the other plant?

First you have to figure out if tall pea plants are dominant over short ones. It should say in the question. I'm assuming it is dominant.
So, as you said, one of the pea plants has the genotype of Tt.
The other genotype of the plant must be either Tt, or TT, because it is tall. If it was a short pea plant, it would have the genotype tt, because it is recessive. If they were to reproduce, you would do a Punnett Square.
You could cross Tt with Tt or TT, depending on whether the other plant was heterozygous or homozygous dominant.
If it was Tt and Tt, the probability of it's offspring would be 25% TT, 50% Tt, and 25% tt. Therefore, 75% would be tall, and 25% would be short.
If it was Tt and TT, it's offspring would be 50% TT, and 50% Tt. Therefore, 100% would be tall.

Hope this helps...

The correct answer is "b"