Final answer:
Staying 'on line' when making vertical entries is required unless it increases the risk of injury or is impossible.
Step-by-step explanation:
According to the information provided, the question seems to be related to safety guidelines while making vertical entries. The answer is (a) 'this would increase entrant risk of injury or is impossible.' When making vertical entries, it is important to stay 'on line,' unless it would increase the risk of injury or if it is impossible to do so. For example, if staying on line would increase the likelihood of falling or if there is no line present to begin with, it is acceptable to deviate from staying on line.