Two heterozygous red flowers. What percentage will the offspring be homozygous dominant?

Only 25% of the offspring will be homozygous dominant.

Genotype: 25% (FF), 50% (Ff), 25% (ff).

Phenotype: 75% red, 25% not red (other color probably)

Percentage: 25% homozygous dominant, 50% heterozygous, and 25% homozygous recessive.

Explanation: Since your question does not mention for any letters, let's assume that F will be the trait for flowers. You can use any letter, so you can change it any time.

The Punnett square will look like this, since we mix two heterozygous red flowers (Ff + Ff)

FF Ff

Ff ff

The percentage that is homozygous dominant will be from the ones that only have F on them twice, which is only 1/4 or 25%.