No.. we have four 5 in 20! as 20! = 1*2*3*4*5*6..*10*11*...15*16..*20, so one five comes from each of 5, 10, 15 and 20. so $5^4$

Which of the following is NOT a factor of 20! ?

A. 17.......$20!=1*2*.....17*...20$.. YES

B. 21=3*7.......$20!=1*2*3.....7*...20$.. YES

C. 25=5*5.......$20!=1*2*...*5*..10*.....*15*....20$.. YES

D. 29.... since 29 is a PRIME number and 20! contains number only till 20, 29 will NOT be there in 20!

E. 42=2*3*7.......$20!=1*2*3*.....7*...20$.. YES

F. 51=3*17.......$20!=1*2*3.....17*...20$.. YES

G. 85=5*17.......$20!=1*2*3*4*5*.....17*...20$.. YES

H. 121=11*11...........$20!=1*2*.....11*...20$.. NO, as we have only one 11.

so D and H

D and H is the answer. Nice question, thanks for adding it!