What material is less rigid than polyether but stiffer than polysulfide?

The correct choice is addition silicone, as it falls within the range of rigidity specified in the question. Addition silicone materials are known for their high dimensional stability and accuracy, making them suitable for various dental applications, including impressions. They possess a rigidity that surpasses that of polysulfide, which is more elastic and forgiving, yet they are less rigid compared to polyether, which is a highly rigid material used for capturing precise details.

In comparing addition silicone to the other materials, irreversible hydrocolloid is known for being less rigid and is primarily used for models due to its flexibility and ease of use, but it lacks the stiffness required for more detailed impressions. Reversible hydrocolloid offers an intermediate option, but is also less rigid than polyether. Alginate is similar in properties to irreversible hydrocolloid, thus making it unsuitable for meeting the criteria in the question. Therefore, addition silicone is the only material that fits the specified range of stiffness.