The correct answer is: C. increases upto a certain limit and then becomes constant.
The yield of most crops increases with the increase in the quantity of water supplied, up to a certain limit. Beyond this limit, the yield will not increase any further, and may even decrease. This is because too much water can lead to problems such as waterlogging, which can suffocate the roots of the plants.
Option A is incorrect because the yield of most crops does not increase continuously with the increase in the quantity of water supplied. Option B is incorrect because the yield of most crops does not decrease continuously with the increase in the quantity of water supplied. Option D is incorrect because the yield of most crops does not increase upto a certain limit and then decrease.