The role of auxin is twofold, that is, low concentration promotes growth and high concentration inhibits growth. The terminal bud can produce auxin, which is transported downward and accumulated in large quantities at the lateral bud, which increases the concentration of auxin at the lateral bud and inhibits the growth of the lateral bud. Therefore, in order to make the hedge surrounded by shrubs grow thick and tidy, gardeners should trim the hedge regularly to break the top advantage and promote the growth of lateral buds.

Therefore, choose: d.