If you can form exactly one meaningful English word using the $1^{\text{st}}$, $5^{\text{th}}$, $6^{\text{th}}$ and $8^{\text{th}}$ letters of the word 'INFORMATION', what will be the fourth letter of that word?

Let's find the selected letters from the word 'INFORMATION':
* $1^{\text{st}}$ letter = $\text{I}$
* $5^{\text{th}}$ letter = $\text{R}$
* $6^{\text{th}}$ letter = $\text{M}$
* $8^{\text{th}}$ letter = $\text{T}$

The available letters are: $\text{I, R, M, T}$.
Arranging these letters to form a meaningful English word yields 'TRIM'.
The letters of 'TRIM' are:
$1^{\text{st}}=\text{T},2^{\text{nd}}=\text{R},3^{\text{rd}}=\text{I},4^{\text{th}}=\text{M}$.

Therefore, the fourth letter of the word is $\text{M}$.