Which sutra is not useful for multiplication:A.     Ekadhikena poorvenaB.     Ekanyunen poorvenaC.     Urdhav tiryagbhyamamD.     Nikhilam

To determine which sutra is not useful for multiplication, we need to analyze each option:

A. Ekadhikena poorvena: This sutra is useful for multiplication as it means "by one more than the previous one." It is commonly used to multiply numbers.

B. Ekanyunen poorvena: This sutra is also useful for multiplication as it means "by one less than the previous one." It is another technique used for multiplication.

C. Urdhav tiryagbhyamam: This sutra is not specifically related to multiplication. It means "vertically and crosswise." It is used for squaring numbers or finding the product of two numbers with a sum of their digits.

D. Nikhilam: This sutra is useful for division, not multiplication. It means "all from 9 and the last from 10." It is used to divide numbers that are close to a power of 10.

Therefore, the sutra that is not useful for multiplication is C. Urdhav tiryagbhyamam.