Potassium, whose outer electron is easily lost, is a highly reactive metal.
(A) Potassium, whose outer electron is easily lost, is a highly reactive metal
(B) Potassium is a highly reactive metal, it has an outer electron that is easily lost
(C) A highly reactive metal, potassium, with an outer electron that is easily lost
(D) The outer election of potassium, a highly reactive metal, is easily lost.
(E) A highly reactive metal that easily loses its outer electron is named "potassium."
The GMAT Sentence Correction is not a test of only grammar: logic is at least as important as grammar. An answer choice that is grammatically correct but logically flawed is always wrong! For a discussion of these ideas, as well as the OE for this particular question, see:
Logical Splits on GMAT Sentence Correction