Explanation:

- The given series is PNNA, RPPE, TRRI, VTTO, ?

- Observe the pattern in the first letters: P, R, T, V. They increase by 2 (P+2=R, R+2=T, T+2=V). The next letter in sequence after V should be X (V+2=X).

- The second letter sequence is N, P, R, T. This also increases by 2. The next letter should be V (T+2=V).

- The third letters are the same as the first: N, P, R, T. Following the pattern, the next should be V.

- The fourth letters sequence: A, E, I, O. They increase by 4 in the English alphabet position (A+4=E, E+4=I, I+4=O). The next should be U (O+4=U).

- All together, the letters form the cluster XVVU.

- Therefore, the correct option is Option: 4 - XVVU.