Melted polymer is applied by a moving nozzle layer by layer, commonly known as 3D printing.

• A. Selective Laser Sintering (SLS)
• B. Stereolithography (SLA)
• C. Direct metal laser sintering (DMLS)
• D. Fusion deposition modelling (FDM)

Answer: (D)

Extrusion-based additive manufacturing, known as fused deposition modelling, uses melted thermoplastic forced through a heated nozzle that moves to lay down material layer by layer. As the nozzle travels, it deposits a bead of molten polymer, and after each layer is completed, the build continues with the next layer on top. This description directly matches the process where melted polymer is applied by a moving nozzle in successive layers, which is the hallmark of FDM. The other methods work differently: laser-based SLS and DMLS fuse powder with a laser rather than extruding melted material, and stereolithography cures a liquid resin with light rather than melting and extruding polymer.