In the following alphabet series, which is $7\text{th}$ letter to the left of $10\text{th}$ letter from your left?

A B C D E F G H I J K L M N O P Q R S T U V W X Y Z

From the word 'ASTOUNDER', what is the maximum number of independent meaningful words that can be made without changing the order of the letters and using each letter only once?

What is the missing letter?

Give the letter in place of the question mark(?).

$A, B, D, ?, K$