The correct answer is option 4: Rotation of the earth.

The rotation of the Earth is known as the Coriolis effect, which causes deflection of winds and ocean currents. In the southern hemisphere, the Coriolis effect causes the wind to be deflected towards the left. This is due to the fact that as the Earth rotates, objects moving from the equator towards the poles are deflected to the right in the northern hemisphere and to the left in the southern hemisphere.

Option 1, difference in the water content of northern and southern hemispheres, is incorrect as water content does not play a role in the deflection of winds.

Option 2, temperature variations, also does not directly cause the deflection of the wind. Temperature variations can influence pressure differences, which can in turn affect wind patterns, but it is not the primary reason for the deflection.

Option 3, difference in pressure, is partially correct as pressure differences do play a role in wind movement, but it is the combination of pressure differences and the Coriolis effect that causes deflection.

Therefore, the rotation of the Earth is the primary reason for the deflection of winds in the southern hemisphere.