Complete the word in the box: Bir[ ]pen. The letters in the box are NOT part of the word.

The pattern involves adding a single letter to the first part of the word (before the box) to form a complete, common English word. The letters after the box are not part of the word being formed.
For example, in number 9, "Kee" + 'p' forms "Keep", and "encil" is extra.

For number 10, the given part is "Bir" followed by a box and then "pen". We need to find a single letter to put in the box that completes "Bir" into a common English word. The "pen" part is extra.

A common English word that starts with "Bir" and is completed by a single letter is "Bird". Therefore, the missing letter is 'd'.

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