A Labrador can be black, yellow, or chocolate brown. What makes it possible to produce

For question a, Labrador retrievers vary in two loci, B and E. Black dogs have dominant alleles at both loci (B-E-), brown dogs have bbE-, and yellow dogs have B-ee or bbee. Since all the puppies were black, a dominant E allele from the brown parent and they must all have inherited a dominant B allele from the yellow parent. The yellow male must have been BBee, and the brown female parent must have been bbEE. The black puppies were all BbEe.

For question b, simply mating yellow with yellow will produce all yellow Labrador puppies. If at least one of the parents is homozygous EE, mating two brown Labradors will produce either all brown puppies, or ¾ brown and ¼ yellow if both parents are heterozygous Ee.