The correct option is A GH78
The first gate has the code AB12.
The second gate has the code CD34.
The third gate has the code EF56.
We can see that the letters and the numbers are placed in ascending order.
The letters in the first three gates are AB, CD, and EF.
So, the letters in the next gate will be GH.
The numbers in the first three gates are 12, 34, and 56.
The pattern can be shown as:
34 – 12 = 22
56 – 34 = 22
So, the number for the fourth gate will be = 56 + 22 = 78.
Therefore, the last gate will have the code GH78.