Which of the following materials is most likely to require immediate pouring to prevent dimensional changes?

Polysulfide polymer is the correct choice because it is known for its elastic properties and the potential for dimensional changes if not handled properly. After being mixed, polysulfide materials undergo a curing process that can be negatively impacted by delays in pouring. The polymerization reaction may not stop immediately after mixing, and if the material sits for an extended period before being poured, it can lead to inaccuracies in the final impression due to shrinkage or distortion.

Other materials listed, such as reversible hydrocolloid, polyether, and addition silicone, have different handling or setting properties. Reversible hydrocolloid can be poured after a delay, as it solidifies by cooling and can be reheated. Polyether can stabilize for a reasonable time before pouring, while addition silicone is less sensitive to immediate pouring and can offer a more forgiving handling time, making them less critical in this regard compared to polysulfide polymer.