The correct answer is D) place the steam pistons in the mid-stroke position. When resetting the slide valves on a duplex reciprocating pump, it is important to first place the steam pistons in the mid-stroke position. This ensures that the slide valves can be properly adjusted and aligned, as the mid-stroke position provides the necessary clearance and access to the slide valve mechanism. Placing the pistons in this position allows for accurate measurement and adjustment of the port openings during the reassembly process. The other answer choices are incorrect because they do not directly address the critical first step of positioning the steam pistons. Measuring the current port openings (option B) and ensuring the balance piston is in the down stroke (option C) are important steps, but they should be performed after the steam pistons are properly positioned. Positioning the steam pistons on 3/4 stroke (option A) does not provide the necessary access and clearance for the slide valve adjustment.