Explanation:

- The summer solstice in the Southern Hemisphere occurs when the tilt of Earth's axis is such that the sun is at its highest point in the sky at noon.

- This event happens around 21st December, marking the longest day and the start of summer in the Southern Hemisphere.

- August 5th, July 18th, and January 11th are not related to the summer solstice in the Southern Hemisphere.

- Option 2 (5th August) and Option 3 (18th July) fall within the Southern Hemisphere's winter months.

- Option 4 (11th January) is in the summer but post-solstice.