please help me in this..c) Goods distributed as free sample Rs 400 were recorded as Rs 4000.

the rectifying entry will be Purchase a/c dr.3600 To Advertising a/c 3600

I wan to ask why the purchase a/c is debited in this Entry

Good attempt Ritu. Although you have provided the correct solution but you missed somewhere in the explanation part of the first entry.

In this problem, Advertising Account and Purchases Account should have been correctly debited and credited with Rs 400 respectively. However, Advertising Account and Purchases Account have been wrongly debited and credited with Rs 400 respectively. This implies that there is an excess debit and excess credit of Rs 3,600. Therefore, in order to rectify this, we should debit Purchases Account and credit Advertising Account with the difference amount i.e. with Rs 3,600.