To choose a letter, I will perform a statistical analysis on the most common letters in the English language. According to a study by Lexico, the following letters are the most frequently used in the Concise Oxford Dictionary. The list below presents the letters in order of frequency, from highest to lowest:

- E - 11.1607%
- A - 8.4966%
- R - 7.5809%
- I - 7.5448%
- O - 7.1635%
- T - 6.9509%
- N - 6.6544%
- S - 5.7351%

Here is a list of possible words that could be the solution:

- Kabukis
- Nabbers
- Oathing
- Rabbets
- Wackier
- Yachted
- Daddies
- Gadgets
- Hackers
- Iambics
- Zarneys
- Xanthic
- Waffled
- Valleys
- Talking
- Salmons
- Rabbits

Additionally, I have the frequency count for each letter:

- a -> 52
- b -> 13
- c -> 15
- d -> 7
- e -> 15
- f -> 3
- g -> 5
- h -> 7
- i -> 15
- j -> 1
- k -> 10
- l -> 13
- m -> 5
- n -> 9
- o -> 3
- p -> 1
- q -> 2
- r -> 11
- s -> 22
- t -> 12
- u -> 3
- v -> 2
- w -> 5
- x -> 2
- y -> 5
- z -> 1

Based on the statistical analysis of letter frequency, the letter "s" is the 7th most common letter in the English language, with a frequency of 5.7351%. It is likely to appear in many words, including 7-letter words where "a" is in the second position, such as "Salmons" and "Rabbits."

Moreover, considering the provided list of words with 7 letters and "a" in the second position, "s" appears in four out of the sixteen examples given ("Kabukis," "Nabbers," "Salmons," and "Rabbits").

Considering both the letter frequency and the examples provided, "s" seems like a reasonable choice.
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