Correct Answer - D
(a) is correct. `6^(th)` coin has four coins on its top which exert a force 4 mg on it.
(b) is correct. `7^(th)` coin has three coins, placed over it. Thus `7^(th)` coin exerts a force 4mg on `6^(th)` coin (downwards)
(c) is correct
As what is explained in (b), the reaction of `6^(th)` coin is 4 mg (upward)
(d) is wrong. `10^(th)` coin, which is the topmost coin, experiences a reaction force of mg (upwards) from all the coins below it.