Pick the correct answer choice for filling the blank in the following sentences.
I was surprised _________.
At his letting out the secret
Why he let out the secret
That he let out the secret
By his letting out the secret
I was surprised at his letting out the secret.
Surprised at is the right use. Therefore at his letting out the secret is the correct. Surprised at (something astonishing, disappointing), surprised by (something unexpected). Both may be used interchangeably in many cases.