Cystic fibrosis is an inherited disease that occurs when an individual is homozygous for a recessive allele. if parents are both heterozygous for this characteristic, what is the probability that their first child will have cystic fibrosis?

25%

Extra information:

Heterozygous means that the individual posesses two different alleles for a particular trait. Homogenous means that the individual posesses two similar alleles.Both parents are heterogeneous, which means that they have a recessive and a dominant allele. If you make a scheme, like the following:
A a
A AA Aa
a aA aa

you can calculate that the chances of the child (doesn't matter whether it is the first or not) has a chance of 25% to have two recessive (aa) alleles and therefore have Cystic Fibrosis.