Option (D): Assertion (A) is false, but Reason (R) is true.
E₁ (less than 3) = {1, 2}, E₂ (greater than 3) = {4, 5, 6}. These are not complementary as they don't cover all outcomes (3 is excluded). Reason (R) is correct: P(E) + P(Ē) = 1.
Complementary events must together cover the entire sample space with no outcome left out. Here, outcome {3} belongs to neither E₁ nor E₂, so their union ≠ sample space, making A false. The Reason is a standard, correct definition from the textbook (Summary point 6), so R is true.