Which section of M.G.L. c. 175 requires an authorized insurer that belongs to an insurance holding company system to register with the Commissioner?

Why this is the answer

M.G.L. c. 175 §206C is the registration section of the Massachusetts holding company provisions (§§206–206D): every authorized insurer in a holding company system registers with the Commissioner, and material intra-group transactions need prior notice or approval. Acquisitions of control fall under §206B.