The correct answer is: C. They are identical but of opposite sign.
A literal is a symbol that represents a number. Two literals are complementary if they are identical but of opposite sign. For example, $2$ and $-2$ are complementary literals, as are $3$ and $-3$.
Option A is incorrect because two literals can be equal without being complementary. For example, $2$ and $2$ are equal, but they are not complementary because they are not of opposite sign.
Option B is incorrect because two literals can be identical and of equal sign without being complementary. For example, $2$ and $2$ are identical and of equal sign, but they are not complementary.
Option D is incorrect because two literals can be unequal but of equal sign without being complementary. For example, $2$ and $3$ are unequal but of equal sign, but they are not complementary.