What specific type of joint must be used for hubless cast iron soil pipe fittings?

The correct answer is the elastomeric sealing sleeve, as it is specifically designed for use with hubless cast iron soil pipe fittings. This type of joint provides the necessary flexibility and sealing capability to accommodate the movement and thermal expansion that may occur in plumbing systems.

Elastomeric sealing sleeves create a secure connection between the pipes while allowing for some degree of flexibility, which is crucial in maintaining integrity in the system under various conditions. These sleeves are engineered to form a watertight seal, ensuring that the system remains leak-free and functions efficiently.

The other options do not suit hubless cast iron soil pipe fittings. Rubber gaskets are typically used with hubbed fittings and may not provide the same level of flexibility as elastomeric sleeves. Metal clamps can assist in securing joints but lack the inherent flexibility and sealing properties required for optimal performance in this specific application, while welded joints are not applicable, as they apply to different types of piping materials altogether. Therefore, the elastomeric sealing sleeve is the preferred choice for ensuring proper connections in hubless cast iron systems.