Answer:
The correct answer would be - 1 out of 4 or 25% offspring would be expected to have yellow flowers
Explanation:
Red flowers are dominant over the yellow flower, initially, a cross was made between true-breeding red and true-breeding of yellow flowers
Red flower plant gametes, a  and a and red plants gamete would be  A and A.
so,
    A      A
a   Aa     Aa
a    Aa     Aa
in this cross, all the offspring are red as they have a heterozygous genotype. Now self-pollinating these F1 generation offsprings, gametes would be A and a for both parents:
    A     a
A Â Â AA Â Â Aa
a    Aa   aa
in the f2 generation, there is only 25% of offsprings are yellow flowers.