The answer was written …… blue ink. A. with B. by C. in D. on

The correct answer is C. in.

The preposition “in” is used to indicate the material or substance that something is made of or contained in. In this case, the answer was written in blue ink.

The other options are incorrect because:

• “With” is used to indicate the instrument or tool that is used to do something. In this case, the answer was not written with blue ink, but in blue ink.
• “By” is used to indicate the person or thing that does something. In this case, the answer was not written by blue ink, but in blue ink.
• “On” is used to indicate the surface or location that something is placed on. In this case, the answer was not written on blue ink, but in blue ink.