Complete the word by filling in the missing letter: 5. Boo [ ] in

In the provided examples (1, 2, 3, 4, 6, 7, 8, 9), a single letter is placed in the box to complete the first part of the word, forming a common English word. The letters following the box are not part of the word being formed.
For instance:

• 1. Car [t] ray $\rightarrow$ Cart
• 2. Truc [k] ey $\rightarrow$ Truck
• 3. Ba [t] ank $\rightarrow$ Bat
• 4. Hous [e] gg $\rightarrow$ House
• 6. Doo [r] ake $\rightarrow$ Door
• 7. Bu [d] am $\rightarrow$ Bud
• 8. Far [m] arl $\rightarrow$ Farm
• 9. Kee [p] e $\rightarrow$ Keep

For number 5, the given part is "Boo" followed by a box and then "in". Following the established pattern, we need to find a single letter to put in the box that completes "Boo" into a common English word. The "in" part is extra and not part of the word being formed.

Common English words that start with "Boo" and are completed by a single letter include "Book", "Boot", "Boom", "Boob", and "Boor". A very common word is "Book". Therefore, the missing letter is 'k'.

The missing letter is $\text{k}$.