In garden peas, tall is dominant to short. if two heterozygous tall plants are crossed, what proportion of the tall progeny will be homozygous dominant?

If you want a visual on this, it is better if you put it in a Punnet square. Lets say that the Tall allele will be represented as "T" and the short as, "t".

If both parents are heterozygous tall then their genotypes would be both Tt, one capital and one lower case. In a Punnet it would look like this:

     T      t 
T  TT   Tt
t   Tt     tt

Homozygous dominant means that the genotype should be TT. If you look at the punnet square, 1 out of the 4 is TT. This means that 25% will be omozygous dominant.