The correct answer is A) allow for expansion without subjecting the joint to flame penetration. The clearance between firebricks in an oil-fired boiler furnace floor is critical to allow for the expansion and contraction of the bricks during heating and cooling cycles. This clearance prevents the joints from cracking or allowing flame penetration, which could damage the surrounding structure and pose a safety hazard. The other options are incorrect because they do not address the primary purpose of the brick clearance, which is to accommodate thermal expansion. Option B is about maintenance, option C is not relevant to an oil-fired boiler, and option D is about filling the joints, not the clearance between the bricks.