G13T, I11R, L9O, _______

Explanation:

First Letter:

• G → I → L → ?
• "G" to "I" is a shift of +2 positions.
• "I" to "L" is a shift of +3 positions.
• Therefore, the next shift should be +4 positions from "L".
• L + 4 = P.

Second Number:

• 13 → 11 → 9 → ?
• 13 → 11 is a shift of -2.
• 11 → 9 is a shift of -2.
• So, continuing the pattern, 9 → 7 (shift of -2).

Third Letter:

• T → R → O → ?
• "T" to "R" is a shift of -2 positions.
• "R" to "O" is a shift of -3 positions.
• Therefore, the next shift should be -4 positions from "O".
• O - 4 = K.

Final Answer: d) P7K.