Doctors hope that one day the body’s master cells, called stem cells, can be directed to grow in organs or tissues appropriate for transplant, use them to test drugs and potentially toxic chemicals, and may study them to gain insight into basic human biology.
Option elimination -
(A) transplant, use them to test drugs and potentially toxic chemicals, and may study them - the reference of them is wrong. It can refer to organs or tissue - wrong.
(B) transplant, using them to test drugs and potentially toxic chemicals, and studied - comma + ING acts as an adverbial and goes back to the main subject (so nothing wrong in the usage) but when compared with C, the parallelism is conveyed better. Moreover, it seems the use of master stem cells to test drugs and potentially toxic chemicals is an important use, putting it in the commas makes it nonessential, which can be eliminated.
(C) transplant, used to test drugs and potentially toxic chemicals, and studied - perfect parallelism.
(D) a transplant, use them for testing drugs and potentially toxic chemicals, and for studying - the reference of them is wrong. It can refer to organs or tissue - wrong. Also, parallelism is broken.
(E) a transplant, used to test drugs and potentially toxic chemicals,and may study them - the reference of them is wrong. It can refer to organs or tissue - wrong. Also, parallelism is broken.