If two individuals who are heterozygous for the sickle-cell disease have children, their offspring: select one: a. will all have the genotype hbs hba. b. will have a one-quarter chance of being a sickle-cell-free homozygote. c. will all be heterozygous for the sickle-cell disease. d. will all have the sickle-cell disease. e. none of the above is correct.

The correct answer is b. will have a one-quarter chance of being a sickle-cell-free homozygote.
Sickle-cell disease is an autosomal recessive disease, which means that if two individuals who are heterozygous for the sickle-cell disease have child, there is  a 25% chance of sickle-cell disease, 25% that the child will not carry any sickle-cell alleles, and 50% that it will have the heterozygous condition (will have an allele for disease).