Final answer:
The SI base unit of length is the meter, redefined in 1983 based on the distance light travels in a vacuum in a fraction of a second. Consequently, a) Meter is the correct answer.
Step-by-step explanation:
The SI base unit of length is the meter (abbreviated as m). Initially defined in 1791 as 1/10,000,000 of the distance from the equator to the North Pole, its definition has evolved to increase precision.
In 1983, the meter was redefined as the distance light travels in a vacuum in 1/299,792,458 of a second. This effectively set the speed of light to exactly 299,792,458 meters per second. The SI base unit of length is the meter (m).
The meter was first defined in 1791 as 1/10,000,000 of the distance from the equator to the North Pole. It has since been redefined multiple times to become more accurate and precise. Currently, the meter is defined as the distance light travels in a vacuum in 1/299,792,458 of a second.
Consequently, a) Meter is the correct answer.