C - Biochemistry is the field that studies the life processes in the laboratory setting to help describe the chemical interactions in play in living organisms.
Incorrect answers:
A - Paleobiology, not biochemistry, is the study of the origin of life on the planet.
B - It would be excessive to claim that descriptions of "all of the reactions that occur in a cell" have been completed. There is much research potential in biochemistry still.
D - Genetic engineering utilizes biochemistry research to modify the characteristics of living organisms but while biochemistry seeks to describe biological processes, genetic engineering attempts to modify them - two complimentary but separate fields of study that both have lots of potential.