Final answer:
A high-pressure boiler plant of 31 to 200 bhp is typically required to be checked regularly to prevent it from being left unattended for more than two consecutive hours.
Step-by-step explanation:
For a high-pressure boiler plant with a horsepower rating of 31 to 200 bhp, the standard practice is to ensure the boiler is checked periodically for safety and efficiency reasons. Regulations and requirements for the attendance of high pressure boilers can vary by location and the complexity of the boiler system. However, generally speaking, such boiler plants are subject to stringent oversight and would typically not be left unattended for extended periods.
While the provided multiple-choice options do not directly correspond to a standardized rule, the requirement that seems most logical based on typical industrial practices is that a high-pressure boiler must be attended to such that it cannot be left unattended over consecutive 2 hours. This ensures a continuous oversight to address any issues quickly and maintain safe operational conditions.