The correct answer is option 4 - "1, 2, and 3."

Harmful algal blooms (HABs) are a phenomenon where certain species of algae grow uncontrollably, leading to adverse impacts on aquatic ecosystems and human health. There are several factors that can contribute to the occurrence of HABs, and the discharge of nutrients from estuaries, run-off from the land during the monsoon, and upwelling in the seas are all recognized as potential causative factors.

Discharge of nutrients from estuaries: Estuaries are areas where rivers meet the sea, and they are often rich in nutrients that are carried downstream from terrestrial ecosystems. When these nutrients are discharged into the sea, they can stimulate the growth of algae and lead to the formation of HABs.

Run-off from the land during the monsoon: During the monsoon season, heavy rainfall can cause nutrient-rich runoff from agricultural land and urban areas to enter the coastal waters. This can provide an additional source of nutrients that can fuel the growth of harmful algae.

Upwelling in the seas: Upwelling is a phenomenon where deep, nutrient-rich water rises to the surface, providing a source of nutrients that can support the growth of algae. While upwelling can be a natural process, it can also be exacerbated by human activities such as overfishing and climate change.

Therefore, the correct answer is option 4 - "1, 2, and 3."