The correct answer is C) expansion joints. Expansion joints are installed in brickwork and masonry structures, including boiler settings, to allow for the expansion and contraction of the materials when heated or cooled. This prevents the brickwork from buckling or cracking due to the thermal stresses. The expansion joints create a flexible connection that accommodates the movement of the materials. The other answer choices are incorrect because they do not specifically address the need to allow for thermal expansion. Insulating bricks or blocks would not prevent the brickwork from expanding, and sliding saddles are not typically used for this purpose in boiler settings.