Mongolism or Down's Syndrome is an autosomal disease caused due to the trisomy of the 21st chromosome as a result of non separation of chromosomes. Trisomy 21 can result from non separation of the chromosomes in the mother , non separation in the father and after the egg and sperm have merged.It is a chromosomal disorder , so it is neither dominant or recessive. As it is an autosomal disorder , so it does not matter whether the male parent or female parent is affected.Thus , if the mother is affected then there are 50% chance of having offsprings , with Down Syndrome as two types of gametes produced by affected mother.

So the correct option is a) 50%
The percentage of offspring produced by an affected mother and normal father would be affected by that disorder by 50%.

hope this helps.