What should I do when I select the wrong option by mistake?

It’s pretty simple, actually; you just need to be more careful. Since you know you are making these types of mistakes, each time you select an answer, just be extra sure that you are selecting the intended radio button.

Hi TheNightKing,

In your initial post in this thread, you actually define WHY that little mistake is happening: you're doing work "in your head." As it stands, that's the WORST way to approach a GMAT question, since it completely removes the proper organization that you'll need (throughout the entire Exam) to maximize your performance. The human brain tends to perform better when it can physically see the work involved - and as an example, the simple act of writing down that the answer to a question is "B" means that you would be far less likely to choose a different answer by accident.

How frequently has this happened to you during a practice CAT/mock? How have you scored on EACH of your CATs/mocks (including the Quant and Verbal Scaled Scores for EACH)?

GMAT assassins aren't born, they're made,
Rich