(D) Assertion (A) is not true but Reason (R) is true.
When two cubes of edge 10 cm are joined, the cuboid formed is 20 cm × 10 cm × 10 cm. TSA = 2(20×10 + 10×10 + 20×10) = 2(200+100+200) = 1000 cm², not 1200 cm². Reason (R) is correct: each face of a 10 cm cube = 10×10 = 100 cm².
When two cubes are joined end-to-end, two faces (one from each cube) are hidden, so the total surface area decreases. Original TSA of two cubes = 2 × 600 = 1200 cm², but joining removes 2 faces of 100 cm² each, giving 1200 − 200 = 1000 cm². The Assertion states 1200 cm², which is wrong. Reason (R) is independently correct (area of each face = 100 cm²), so answer is (D).