The letter was written . . . . . . . . red ink. A. with B. by C. in D. do

The correct answer is: C. in

The word “in” is used to indicate the medium or material with which something is made or done. In this case, the letter was written in red ink.

The other options are incorrect because:

• “With” is used to indicate the instrument or tool with which something is done. In this case, the letter was not written with red ink, but in red ink.
• “By” is used to indicate the person or agent who does something. In this case, the letter was written by someone, but the question does not ask who wrote it.
• “Do” is a verb that means to perform an action. In this case, the letter was not performed, but written.