by Sage Pearce-Higgins Sat Aug 08, 2020 6:54 am
That's an interesting sounding rule, and one I haven't heard before. I can only find one example of the use of 'for example' in an official GMAT problem, that's SC 813 from OG2020. From my experience, I can say that GMAT favors the phrase 'such as' - check out problems 900, 855, and 764 from OG2020.
It's worth remembering that GMAT doesn't publish the "rules" that it adheres to. Instructors (like me) analyze GMAT problems and try to see patterns in the phrases and grammatical constructions that they follow. There are many types of English, and I know that the way I speak and write English doesn't always line up with the way that GMAT uses English. They tend to write sentences that are somewhat conservative, rather formal, and very precise in meaning. However, take heart that you're not being expected to memorize a bunch of random rules: if you focus on the meaning of the sentence, the modifiers, verb tenses, and comparisons, the grammar of subject-verb and pronouns, and the construction of parallel phrases, then you'll be able to eliminate incorrect answer choices.