If all the letters of the word RATIONALISATION are written according to the alphabetical order, which letter will be farthest from the first letter?

"Farthest from the first letter" means the letter that will sit at the very end (the last position) when all letters are arranged from A to Z. This is simply the alphabetically largest letter present in the given word.
Let's extract all the unique letters present in the word RATIONALISATION:

$$\text{R, A, T, I, O, N, L, S}$$

Let's look at their corresponding alphabetical ordering positions ($A=1,Z=26$):

* $\text{A}=1$
* $\text{I}=9$
* $\text{L}=12$
* $\text{N}=14$
* $\text{O}=15$
* $\text{R}=18$
* $\text{S}=19$
* $\text{T}=20$

Since T has the highest alphabetical positional value among all letters present in the word, it will occupy the final spots at the rightmost end. Hence, T is farthest from the first letter.