Answer:
D. Most infants died before childhood.
Explanation:
Unfortunately, most children fell to disease and harsh conditions. Back then medicine was not evolved as it is now and most would die of the common cold. But it was also customary to have large families in order to support the family so kids often were used as labor and died in unfortunate accidents or from exhaustion. Today, there are many many laws protecting children from such things and medicine has evolved so most people in wealthy countries could live up to 70 years old if they take care of themselves.
The option that best describes the cause-and-effect structure in a memoir is when the structure focuses on an incident and its consequences.
I would say that the correct option is the third one.
The cause is the incident, and the effect are its consequences.