If the letters in the word $\text{CATASTROPHE}$ are rearranged in alphabetical order, which one will be in the middle after the rearrangement?

The given word is $\text{CATASTROPHE}$.
Total number of letters = $11$

Arranging the letters in alphabetical order:
$\text{A, A, C, E, H, O, P, R, S, T, T}$

Since there are $11$ letters, the middle position is the $6^{\text{th}}$ letter:

$$\text{Middle position}=\frac{11+1}{2}=6^{\text{th}}\text{letter}$$

Counting to the $6^{\text{th}}$ letter in the sorted sequence:
$1^{\text{st}}:\text{A}$
$2^{\text{nd}}:\text{A}$
$3^{\text{rd}}:\text{C}$
$4^{\text{th}}:\text{E}$
$5^{\text{th}}:\text{H}$
$6^{\text{th}}:\text{O}$

The letter in the middle is $\text{O}$.