Given, N + D + R = 120 ....... (1)

(1) The sum of the amounts of money that Nadia and Dan have is half the amount that Rachel has.
--> N + D = R/2
From (1), R/2 + R = 120
--> 3R/2 = 120
--> R = 80
--> N + D = 80/2 = 40
--> We cannot find the value of N --> Insufficient

(2) The sum of the amounts of money that Nadia and Rachel have is five times the amount that Dan has.
--> N + R = 5D
From (1), 5D + D = 120
--> 6D = 120
--> D = 20
--> N + R = 100
--> We cannot find the value of N --> Insufficient

Combining (1) & (2),
N + D = 40 & N + R = 100
Adding, N + D + N + R = 140
--> N + 120 = 140
--> N = 20 --> Sufficient

IMO Option C