What was the cause of Helen Keller's disability?

The question refers to "Helen's disability." This is a common reference to Helen Keller, who became blind and deaf at a young age.

Helen Keller became ill at 19 months old with an unknown disease, possibly scarlet fever or meningitis, which left her blind and deaf. The illness was characterized by a high fever.

a. accident - Incorrect. Her disability was due to illness, not an accident.
b. heredity - Incorrect. Her disability was not inherited.
c. high fever - Correct. Her illness involved a high fever that led to her disabilities.
d. genetics - Incorrect. Her disability was not genetic.

The correct option is c.

The cause of Helen Keller's disability was a severe illness that resulted in a high fever.

The final answer is $\boxed{\text{c. high fever}}$.