Final answer:
In rebuilding a clutch pack for an automatic transmission, clean transmission fluid should be used on the piston seals during assembly. This ensures the seals are properly lubricated for correct installation and to avoid damage.
Step-by-step explanation:
When rebuilding a clutch pack in an automatic transmission, the piston seals should be lubricated during assembly to ensure proper installation and operation. The correct substance to use is b. Clean transmission fluid. Applying clean transmission fluid to the seals helps to prevent them from drying out, ensures they seat properly, and prevents damage such as tearing or distortion during the installation process. It is not recommended to use engine oil, solvent, or to install the seals dry, as these options do not provide the same benefits and could lead to improper assembly and future transmission issues.