Option 4, which states that the correct sequence of ecosystems in the order of decreasing productivity is Mangroves, grasslands, lakes, and oceans, is the correct answer.

This means that mangroves are the most productive ecosystem, followed by grasslands, lakes, and oceans, with oceans being the least productive among the four options.

Mangroves are highly productive ecosystems due to the abundance of sunlight, rich organic matter, and nutrient availability. Grasslands also have high productivity, as they support a diverse range of herbivores. Lakes have lower productivity compared to mangroves and grasslands, as they rely on external sources of nutrients and sunlight. Oceans have the lowest productivity as they are vast and often nutrient-poor, with limited sunlight penetration.

It is important to note that the productivity of ecosystems may vary depending on various factors such as geographical location, climate, and nutrient availability.