Frontal rain is caused by option 3, which is cyclonic activity. Cyclonic activity refers to the movement and interaction of air masses of different temperatures, usually associated with low-pressure systems. When a warm air mass and a cold air mass meet, a front is created. As the warm air rises above the cold air, it cools and condenses, forming clouds and precipitation. This is known as frontal rain.

Option 1, convection currents, are responsible for creating thunderstorms and localized rain showers, but not frontal rain. Option 2, winds from the sea, can bring moisture but alone cannot cause frontal rain. Option 4, condensation of water evaporated from mountains, can lead to orographic rainfall, but not frontal rain. Therefore, option 3 is the correct cause of frontal rain.