The question asks us to identify the factor that causes a change in the usual direction of ocean currents in the Indian Ocean.

Option 1 suggests it`s because Indian Ocean is ‘half an ocean’. This is a confusing description, but it refers to the fact that the Indian Ocean is hemmed in by land on three sides. However, this geographical feature doesn`t alter the current`s direction.

Option 2 says "Indian Ocean has monsoon drift" and this is the correct answer. The monsoon, which brings seasonal winds, significantly influences the direction of ocean currents. During the summer monsoon, winds blow from the sea towards the land, while in winter they blow from the land towards the sea, causing ocean currents to change direction.

Option 3 claims that it`s because the Indian Ocean is `land-locked`. This is misleading, as, while the Indian Ocean is surrounded by land on three sides, it`s not completely land-locked and doesn`t significantly alter the direction of its currents.

Option 4 suggests that the increased variation in salinity causes the shift. Ocean currents can indeed be influenced by differences in salinity, but this does not account for the regular, seasonally fluctuating directional change seen in the Indian Ocean.