What is the pattern of inheritance for a rare recessive allele [a] every affected person has an affected parent. [b] unaffected parents can produce children who are affected. [c] unaffected mothers have affected sons and daughters who are carriers. [d] none of the above?

I am sure that the pattern of inheritance for a rare recessive allele is such that unaffected parents can produce children who are affected. 
Most genetic diseases are caused by very rare recessive alleles. It is extremely unlikely tat an individual will inherit two copies of the recessive allele and therefore, their incidence in the population is very small. Examples of the genetic diseases include Cystic fibrosis and Huntington's disease both caused by recessive genes.