In certain plants, tall is dominant to short. if a heterozygous plant is crossed with a homozygous tall plant, what is the probability that the offspring will be short?
a) 1
b) 1/2
c) 1/4
d) 1/6
e) 0

The answer is E) 0.

If:
A - dominant allele,
a - recessive allele,

then:
AA - dominant homozygous (tall),
aa - recessive homozygous (short),
Aa - heterozygous (tall).

A heterozygous plant (Aa) is crossed with a homozygous tall plant (AA):

Parental generation:         Aa     x    AA
F1 generation:           AA     AA     Aa     Aa

All of the offspring is tall (AA or Aa), since tall is dominant to short.
There is no chance for offspring to be short (aa).