Answer:
B. Proteins are tethered to the extracellular matrix
Explanation:
The assembly of nascent focal adhesions is strongly dependent on the process of actin flow, happening in migrating cells when the actin filament polymerize at the edge and flow back towards the same cell's body.
That is what causes the traction needed for migration, when the focal adhesion works a support when it tethers to the extracellular matrix and impedes the backward move of actin, generating the traction force needed at the point of contact to move the cell forward.