Question

Fill in the blank with the most appropriate word:
"Why did you open my letter?"
"I'm sorry. I did it ____ mistake."

• A. with
• B. at
• C. by
• D. on

Answer 1

Answer: (C)

by

Answer : C

Prepositions are words that are used to show relation of a noun to another element (noun/adjective/verb) in the sentence.

Prepositions of Agency or Instrumentality are used to link a person or thing to the action/activity that it does.

Let's take a look at the options:

• with : It is usually used to show a doing by a thing or object
• at : can work as a preposition of time and place but NOT as a preposition of agency
• by : It is usually used to show a doing by a person but can also be used for a thing or object
• on : can work as a preposition of time and place but NOT as a preposition of agency

As here the doer is a person ("I"), the most suitable option is "by".

Thus, the correct complete sentence is:

"I'm sorry. I did it BY mistake.