If the first and second, third and fourth, fifth and sixth and so on. letters of the word ‘FORMULIZED' are interchanged, then the ninth letter from the left will be:

Instruction: Interchange the first and second, third and fourth, fifth and sixth, seventh and eighth, and ninth and tenth letters of the word.

After Interchanging the pairs:

First and second letters: F and O → OF

Third and fourth letters: R and M → MR

Fifth and sixth letters: U and L → LU

Seventh and eighth letters: I and Z → ZI

Ninth and tenth letters: E and D → DE

After interchanging the letters in pairs, the word becomes: ​OF MR LU ZI DE

The ninth letter from the left is D.

Thus, the correct option is (d).