Where a service mast is used for the support of service drop conductors, it shall be of adequate strength or be supported by ___.

The correct response focuses on the requirement for the service mast to have sufficient strength or be adequately supported. In this context, using braces or guy wires provides structural support to the service mast. These supports help to stabilize the mast against lateral forces, such as wind or the weight of the service drop conductors themselves. Bracing and guying are standard practices in electrical installations to ensure that masts remain upright and secure, thereby reducing the risk of failure or collapse.

While other options are considered in various scenarios, such as using heavy rock ballast or concrete anchors, they do not provide the same level of support specifically designed for the lateral stability needs of a service mast. Additionally, additional cables could refer to extra conductors rather than structural support. Thus, braces or guy wires are the most straightforward and reliable means to ensure the necessary support for a service mast.