In a post office stamps of three denominations of Rs. 7, Rs. 8, Rs. 10 are available. The exact amount for which one cannot buy stamps is:

Correct option is A

According to the option,

Rs29 = 3(Rs7  Stamp)+ 1(Rs8  Stamp)

Rs23= 1(Rs7 Stamp) +2(Rs8 Stamp)

Rs20= 2 (Rs10  Stamp)

Here, clearly we can see, 

for Rs 19 one cannot buy any stamps.