The correct option is Heating by solar energy causing water expansion..
Explanation
The variation in sea level across different latitudes is primarily influenced by the physical properties of seawater, specifically density changes driven by temperature.
Detailed Analysis:
- Thermal Expansion: The equatorial region receives the most direct and intense solar insolation throughout the year. This results in significantly higher sea surface temperatures compared to the middle and high latitudes.
- Volume Increase: As water warms, its density decreases, and it undergoes thermal expansion. This means that a column of warm water occupies a greater volume than an equivalent mass of cold water.
- Sea Level Rise: Due to this expansion, the ocean surface at the equator stands approximately 8 cm higher than in the middle latitudes. This slight gradient contributes to the flow of ocean currents as water tends to flow down the slope.
Key Takeaway: Intense solar heating at the equator causes thermal expansion of seawater, resulting in a slightly higher sea level compared to cooler latitudes.