Which unit is used as the base unit of time in the International System of Units (SI)?

The base unit of time in the International System of Units (SI) is the second. The second has been defined in terms of specific physical phenomena, such as the vibrations of a cesium atom, which provides a high level of precision. This definition allows for a consistent and standardized measurement of time across various scientific disciplines and applications.

The other options, like minutes, hours, and days, are derived units based on the second; for instance, a minute consists of 60 seconds, an hour has 3600 seconds (60 minutes), and a day comprises 86,400 seconds (24 hours). These derived units are useful for everyday timekeeping, but in scientific contexts, the second is favored for its foundational status within the SI system.