Question should be What is the difference in probability that no one in a family of 10 people have that birth defect.
Use binomial distribution (Bernoulli trials). S=0.9, F=0.1 (S represents successful (non-defect) birth; evaluate the expansion of (S+F)10 using the binomial theorem. The term that represents no F results is S10; all other terms have F as a factor, to some exponent, along with a binomial coefficient.
Answer is 0.910 = 0.3487 rounded to 4 decimal places.