Many ancient civilizations were adept at observing celestial bodies, predicting astronomical events, and for developing complex calendars. Which option corrects the grammatical error in the underlined portion of the sentence?

The correct answer to the question "Many ancient civilizations were adept at observing celestial bodies, predicting astronomical events, and for developing complex calendars. Which option corrects the grammatical error in the underlined portion of the sentence?" is Option B: developing complex calendars..

The other options — A) for developing complex calendars., C) they developed complex calendars., D) the development of complex calendars. — are incorrect for this GMAT Exam Prep question. As explained above: Option B correctly maintains parallelism with the preceding gerunds 'observing' and 'predicting.' The phrase 'adept at' should be followed by a series of parallel gerunds.