The correct option is B Balancing on skis while moving sideways and forward.
In the passage, the writer mentions that the learners tried to master the art of balancing on the skis. So, the answer is option B. There is no mention of avoiding bumping into toddlers, so option C is incorrect. Options A and D are not the difficult part of learning to ski for beginners specifically - A is too easy and D is possibly for more advanced learners. Hence, options A and D are also incorrect.