Step 1: State the concept.
Aerosil is a well-known trade name for a pharmaceutical excipient used mainly as a glidant to improve the flow of powders during tablet and capsule manufacture.
Step 2: Recall its chemical identity.
Chemically, Aerosil is colloidal silicon dioxide, commonly called colloidal silica. Its very fine particles coat granules and reduce friction, improving flowability.
Step 3: Eliminate spray drying lactose.
Option 1 is wrong because spray dried lactose is a directly compressible filler or diluent, a different excipient with a different role.
Step 4: Eliminate calcium sulphate.
Option 2 (calcium sulphate) is a diluent and is chemically unrelated to Aerosil, so it is rejected.
Step 5: Eliminate HPMC.
Option 4 (HPMC, hydroxypropyl methylcellulose) is a cellulose-based binder, film former, and release modifier, not a colloidal silica, so it is discarded.
Step 6: Conclude.
Aerosil is also called colloidal silica, which is option 3.
\[ \boxed{\text{Option 3: Colloidal silica}} \]