Non-normative influences are unique, irregular events that have a significant impact on an individual's life but do not typically occur in the typical life course that most people experience. In your examples:
a. A man in late adulthood who experiences problems with visual acuity - this is more of a normative influence as vision problems can be common in late adulthood.
b. A young boy who loses his leg in a car accident - this is a non-normative influence as it is a unique event that does not happen to everyone.
c. A 20-year-old woman who develops breast cancer - this is also a non-normative influence because developing breast cancer at such a young age is rare and affects her life course significantly.
Therefore, the correct answer is d. b and c, both represent non-normative influences.