The correct answer is A) Strip the board of all nonvital circuits. When a diesel generator trips off the board due to a mechanical failure, the first priority is to protect the electrical system from potential damage. Stripping the board of all nonvital circuits helps to reduce the electrical load and prevent the remaining generator from becoming overloaded, which could lead to further system failures. This action is typically the first step in responding to a diesel generator failure in a parallel operation scenario. The other options, while important, are not the immediate first step. B) Securing alarms and restarting the tripped generator should only be done after the electrical system is stabilized. C) Ascertaining the cause of the low lube oil pressure is necessary but comes after the initial emergency response. D) Starting the emergency generator may be required, but this should be done after the main system is secured.