Step 1: Understand the requirement.
An antiseptic used in soap must be effective against microorganisms and suitable for regular skin contact.
Step 2: Analyze the options.
Iodoform and tincture iodine are used for wound dressing, not in soaps.
Boric acid is a mild antiseptic but not commonly used in soaps.
Step 3: Identify the correct compound.
Bithional is an antibacterial compound commonly added to medicated soaps.
Step 4: Conclusion.
Therefore, bithional is used as an antiseptic in soap.