Answer :

C

Explanation :

Start with the A in the bottom left corner, and move in a clockwise spiral around the diagram towards the centre. Letters are written in alphabetic order, in steps of 6 letters at a time.

Answer :

X

Explanation :

Start at the top left and move in alternate boxes from left to right, then down one row and to the left, in a snakes and ladders pattern. Letters advance through the alphabet in steps of 2, 3, 4...8, before repeating this pattern.

Answer :

T

Explanation :

Working through the diagram in columns, left to right, The sum of the numerical values of the letters in each column follows the sequence 17, 18, 19 and 20.

Answer :

T

Explanation :

In each row, add together the numerical values of the pairs of letters to the left and right, then calculate their difference, putting the letter with this numerical value in the centre space.

Answer :

M

Explanation :

Working in rows, from top to bottom, the sum of the numerical values of the letters follows the sequence 16, 18, 20, 22, 24.