Which phrase listed below best describes the function of the digestive system?
a. to filter out waste from the blood
b. to remove waste from the body
c. to break down food into nutrients
d. to protect the body against germs
b?

C is the correct answer. The main function of the digestive system is to break down food into nutrients.

Food is needed to fuel the body for energy, and also for growth and repair. The digestive system converts the food we eat into their simplest forms , like glucose, amino acids and fatty acids. The broken down food is then absorbed into the bloodstream from the small intestine and the nutrients are carried to each cell of the body.

The large intestine  serves as a storage organ for the excretory products. However though this is true, the main function of the digestive system is to break down food into nutrients. The excretory duty is more of a necessity rather than a primary function.

the last one: artificial selection/genetic engineering

plant breeding is modifying the genetic composition of the plant - but this isn't cloning (which is specifically introducing a gene into a genome). but you are artificially selecting for the phenotype that you want.

the second one is literally genetic engineering - the gene would have been cloned first and then inserted or genetically engineered into the new plant cell (the cotton).