Sentence Correction

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.
A. transplant, use them to test drugs and potentially toxic
chemicals, and may study them
B. transplant, using them to test drugs and potentially toxic
chemicals, and studied.
C. Transplant, used to test drugs and potentially toxic chemicals,
and studied.
D. A transplant, use them for testing drugs and potentially toxic
chemicals, and for studying.
E. A transplant, used to test drugs and potentially toxic chemicals,
and may study them.
[spoiler]A/C[/spoiler]

IMO C

among all I would also chose C because in A [them] can refer to cells and doctors

C for parallelism of participles

stem cells can be directed to...., used to test and studied to gain...

IMO C...

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 ... study them to gain insight into basic human biology.

C:

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, used to test drugs and pote ... studied. to gain insight into basic human biology.

C for parallelism - directed, used, and studied