One-time-pad possible solutions
$(i,j,k)$ is not a specific OPT. It's three variable names that you can use to talk about the key. The question is just asking, "Given all possible three-character OTPs, how many valid English plaintexts are there for the ciphertext CJU?"

In other words, how many different values are there for the triple $(i,j,k)$ such that "CJU" decrypts to an actual English word.