Which is the most likely reason that stem cell research is controversial?

The correct answer is that it involves human embryonic stem cells (ESC).

Explanation:

Stem cells are regenerative and differentiating cells which can develop into any type of cells even after they remain inactive for a long time. They are the first differentiation cells of embryo proper which are formed just after the fertilization.

The research on these Embryonic stem cell (ESC) is a reason for legal and ethical issues which led to the social controversy after the first successful culturing of human  ESCs in the laboratory in 1998 by James Thomson and John Gearhardt as "it destroys the embryos to obtain the pluripotent ESCs.

Thus, it involves human embryonic stem cells (ESC) is the correct answer.

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