Step 1: Recall the basic antibody unit.
Every antibody is built from a basic Y-shaped unit made of two heavy chains and two light chains. On its own, this single unit is called a monomer.
Step 2: Ask which antibodies stay as a single unit.
IgG, IgD, and IgE never join up with other units using a joining (J) chain. Each one works alone as one Y-shaped monomer.
Step 3: Check which antibodies join into bigger units.
IgA usually links two monomer units together with a J chain to form a dimer, especially in secretions like saliva and tears. IgM links five monomer units with a J chain to form a large pentamer, which is why it works so well early in an infection.
Step 4: Match this back to the answer choices.
Any option that includes IgA or IgM is including a non-monomeric antibody, so those options are wrong. Only the option with IgD, IgE, and IgG has all three staying as single Y-shaped units.
Final Answer:
The purely monomeric antibodies are IgD, IgE, and IgG, option (4).