A billionth of a second is defined as a:

The correct answer is: C. nanosecond

A nanosecond is one billionth of a second. It is a unit of time used to measure very short periods of time, such as the time it takes for light to travel across a DNA molecule.

A millisecond is one thousandth of a second. A microsecond is one millionth of a second. A picosecond is one trillionth of a second.

None of the above options are correct.