The correct answer is B) check the lube oil temperature. The most practical method of determining the condition of a shaft bearing while the shaft is in operation is to check the lube oil temperature. This is because an increase in the lube oil temperature can indicate increased friction and wear within the bearing, which can be an early indicator of bearing issues. Checking the lube oil temperature is a simple and effective way to monitor the condition of the bearing without having to disassemble the equipment. The other options are not as practical or effective for monitoring bearing condition while the shaft is in operation. Option A, performing a carbon blot test on an oil sample, would require taking an oil sample and sending it for laboratory analysis, which is more time-consuming. Option C, checking the lube oil viscosity, would also require additional testing that may not be as readily available. Option D, visually inspecting the bearing, is not practical while the shaft is in operation, as the bearing would need to be accessed and inspected directly.