In a simple mendelian cross, true-breeding tall plants (tall is dominant) are crossed with short plants (short is recessive), and the f1 plants, which are all tall, are allowed to self-pollinate. what fraction of the f2 generation are tall?

Considering that T is for the dominant allele

considering that t is for the recessive allele

by test cross you'll get

TT, Tt, Tt, tt

since T is the dominant allele, Tt would mean that the plant is also tall

hence the ratio would be 1:3 (short:tall)