The correct answer is option 4: Due to the effect of Coriolis force. The Coriolis force is caused by the rotation of the Earth. As the Earth spins on its axis, the horizontal motion of objects is deflected to the right in the Northern Hemisphere and to the left in the Southern Hemisphere. This deflection is what causes the wind to blow in a curved path instead of a straight line.

In the case of the summer monsoon winds in the Northern Hemisphere, the Coriolis force causes the winds to blow from the south-west. As the landmass of India heats up during the summer, it creates a low-pressure area. The surrounding areas with higher pressure, such as the Indian Ocean, have air moving towards the lower pressure. However, due to the Coriolis force, this air is deflected to the right in the Northern Hemisphere, resulting in the south-western direction of the summer monsoon winds.

Option 1 is partially correct as the general direction of wind from the Indian Ocean is indeed south-western, but it does not explain why this is the case.

Option 2 is not correct as the presence of the doldrums around the Equator is not directly related to the direction of the summer monsoon winds.