Final answer:
Proteins expected to bind to the 'whiskers' of formin, enhancing actin polymerization, are Profilin, as it helps in adding actin monomers to the growing filaments.
Step-by-step explanation:
The actin-nucleating protein formin has binding sites on its whiskers, which are involved in the recruitment of actin subunits to facilitate polymerization. The protein expected to bind to these sites to promote this process is Profilin. Profilin is known to bind to actin monomers (G-actin) and contributes to the polymerization of actin by aiding the addition of these monomers to the growing filamentous actin (F-actin). This is in contrast to other actin-binding proteins like thymosin, which sequesters actin monomers, cofilin, which disassembles actin filaments, gelsolin, which severs filaments, tropomodulin, which caps the minus end of actin filaments, and tropomyosin, which stabilizes actin filaments in muscle cells. Hence, the answer is B. Profilin.