In my opinion the answer is C
are able to vs can be – “are able to” implies that stem cell have the ability to perform the task itself. While “Can be” implies that stem cell can be programmed (but it will not program itself)
A) are able to be programmed to behave like pancreatic cells
Reason to eliminate: are able to is not the correct usage as per the logical meaning and reason discussed above
B) were able to be programmed to behave as pancreatic cells
Reason to eliminate: “were” is not the right tense to describe a general statement
C) can be programmed to behave like pancreatic cells
Correct choice: Can be is the right usage
D) could be programmed to behave like pancreatic cells do
Reason to eliminate: We can argue that can be is better fit than could be, but for me the biggest reason to eliminate this choice is “do”. If we replace do with the verb it’s referring to the sentence becomes – Stem Cells could be programmed to behave like pancreatic cells could be programmed.
Now this is an incorrect comparison/meaning, the original meaning that needs to be communicated is that the stem cell can be programmed to behave like pancreatic cells
E) are capable of being programmed to behave like pancreatic cells do
Reason to eliminate – We have issues with usage of “being” here, but I would eliminate this choice for the same reason as option D – Usage of “D”