The latitude of a place refers to its angular distance from the Equator. The Equator is a fundamental reference line that circles the Earth at its widest point, dividing it into the Northern Hemisphere and the Southern Hemisphere. Latitude measurements are expressed in degrees, with 0 degrees being at the Equator and increasing towards the North Pole and decreasing towards the South Pole.

Option 1 (Equator) is the correct answer because it accurately identifies the reference line used to express latitude. Option 2 (South Pole) and option 4 (North Pole) are incorrect because they represent points on the Earth`s surface rather than reference lines. Option 3 (Axis of the Earth) is also incorrect because the latitude is measured in relation to the Equator, not the Earth`s axis.