Choose the missing terms out of the given alternatives : R, U, X, A, D, ?

The correct answer is $\boxed{\text{I}}$.

The sequence is formed by taking the next letter in the alphabet, three places after the previous letter.

• R
• U (3 places after R is U)
• X (3 places after U is X)
• A (3 places after X is A)
• D (3 places after A is D)
• I (3 places after D is I)