Answer:
1/4 or 25% probability of 3 brown kittens.
Explanation:
Black fur is dominant = B
Brown fur is recessive = b
Heterozygous cats = Bb
Cross between two heterozygous cats
Bb x Bb
The result of that genotype will be BB, Bb, Bb, bb
BB = 1/4 or 25%
Bb (2) = 2/4 or 50%
bb = 1/4 or 25%
So, brown fur (bb) has 25% chance in three kittens.