Actin is which type of filament?

• A. Myosin
• B. Titin
• C. Actin
• D. Nebulin

Answer: (C)

Actin is a microfilament, the thinnest type of filament in the cytoskeleton. Microfilaments are made from actin monomers that polymerize into long, flexible strands about 7 nanometers in diameter. In muscle, these thin filaments form the actin-containing portion of the sarcomere’s thin filament, where they slide past thick myosin filaments to produce contraction with the help of regulatory proteins like tropomyosin and troponin.

The other options refer to different components: myosin is the motor protein that forms thick filaments and drives contraction; titin is an elastic protein that spans from Z-lines to M-lines helping maintain structure and elasticity; nebulin helps organize and stabilize actin filaments. So the correct concept is recognizing actin as the filament that constitutes microfilaments (thin filaments) of the cytoskeleton.