According to your text, why do people in low-income countries have much lower life expectancies than those in high-income countries?

You did not have given the options. Right option is:

d) Malnutrition and sanitation in poor countries leads to generally lower health making people more susceptible of fatal diseases and more likely to die from diseases that are not usually fatal.

Explanation

About 36 countries have malnutrition. It's very hard for some people of low income countries to afford the food, which leads them to the diseases. Again the medical services of those countries are not good enough to provide the better treatment. That's how people are more susceptible of fatal diseases due to the poor sanitation and malnutrition. Which results in the increase of death rate from diseases that are not even fatal. They don't even have the vaccines in case of outbreak, like ebola virus has taken more then 11,000 lives in west Africa.