The correct answer is B) Tighten the gag only finger tight to prevent damage to the valve stem, disc or seat. When gagging a boiler safety valve, it is crucial to avoid overtightening the gag, as this can damage the delicate components of the valve. Tightening the gag only finger-tight ensures that the valve stem, disc, and seat are not subjected to excessive force, which could compromise their integrity and prevent the safety valve from functioning properly when needed. The other options are incorrect because: A) using a special wrench is not necessary, as finger-tight is sufficient; C) checking the free movement of the safety valve parts is important, but not part of the gagging process; and D) the gag should make contact with the safety valve stem to securely hold it in place.