The correct answer is B) fluid level in the mud pits decreases rapidly. On a MODU (Mobile Offshore Drilling Unit), a rapid decrease in the fluid level in the mud pits is an obvious indicator of lost circulation of drilling fluid. This is because lost circulation occurs when the drilling fluid escapes the borehole and is lost into the surrounding formation, rather than returning to the mud pits. As the fluid is lost, the level in the mud pits will decrease rapidly. The other options are incorrect because they do not directly indicate lost circulation. A decrease in cuttings on the shale shaker (option A) or an increase in cuttings (option C) may be related to lost circulation, but are not direct indicators. An increase in fluid level in the mud pits (option D) would indicate the opposite of lost circulation.