What is Greenwich Mean Time (GMT) primarily used for?

Greenwich Mean Time (GMT) serves as a fundamental reference for standard timekeeping globally. It is primarily used to ensure a uniform framework for time across different regions, which is especially critical in managing international communications, transport, and business operations. By establishing GMT as a baseline, stakeholders can coordinate activities across various time zones, facilitating everything from flight schedules to international meetings and trade.

While other options may relate to time or financial practices in varying contexts, they do not directly reflect the primary function of GMT. For instance, the measurement of solar time pertains more to local timekeeping and not to the standardized global approach that GMT provides. Similarly, the determination of discount rates and interest rates pertains to financial metrics rather than timekeeping. Thus, the utility of GMT as a universal time standard is what sets it apart as the most relevant answer to this question.