Step 1: State the concept.
In topical products like creams and lotions, certain ingredients are added simply to keep the preparation and the skin from drying out. A substance that holds moisture is called a humectant.
Step 2: Identify glycerin's nature.
Glycerin (glycerol) is a hygroscopic polyol, meaning it attracts and retains water. This makes it ideal for preventing moisture loss.
Step 3: Reason about its external use.
Applied externally, glycerin draws water and keeps the skin hydrated while stopping the formulation from hardening as water evaporates. That is exactly the humectant role.
Step 4: Eliminate therapeutic agent.
Option 1 is wrong because glycerin here is an excipient that supports the formulation, not the active drug treating a disease.
Step 5: Eliminate surface active agent and antiseptic.
Option 3 (surface active agent) is wrong because glycerin is not a surfactant; it does not lower surface tension significantly. Option 4 (antiseptic) is wrong because glycerin is not used to kill microbes at the concentrations used here.
Step 6: Conclude.
Glycerin is used as a humectant in external preparations, which is option 2.
\[ \boxed{\text{Option 2: Humectant}} \]