What is the tension in the string connecting block A and B?

Answer: Option C

For such questions, always look at the blocks as one single system and find the acceleration of the whole as one. That acceleration is the same for both blocks.

Then apply free body diagram (the one with the unknown T and with the least number of forces acting), find the unknown T.

Refer to this post for more examples of such questions (some involve friction)

https://www.sgphysicstuition.com/post/finding-forces-on-multiple-bodies