The correct answer is A. N = y + R.
Porosity (N) is the ratio of the volume of voids to the total volume of a rock or soil. Specific yield (y) is the volume of water that a unit volume of saturated rock or soil will yield by gravity. Specific retention (R) is the volume of water that a unit volume of saturated rock or soil will retain against gravity.
The relationship between porosity, specific yield, and specific retention can be expressed as follows:
N = y + R
This equation states that the total volume of voids in a rock or soil (porosity) is equal to the volume of water that the rock or soil will yield by gravity (specific yield) plus the volume of water that the rock or soil will retain against gravity (specific retention).
Option B is incorrect because it states that specific yield (y) is equal to the sum of porosity (N) and specific retention (R). However, specific yield is only equal to the volume of water that a unit volume of saturated rock or soil will yield by gravity, not the total volume of voids in the rock or soil.
Option C is incorrect because it states that specific retention (R) is equal to the sum of porosity (N) and specific yield (y). However, specific retention is only equal to the volume of water that a unit volume of saturated rock or soil will retain against gravity, not the total volume of voids in the rock or soil.
Option D is incorrect because it states that specific retention (R) is greater than the sum of porosity (N) and specific yield (y). However, specific retention is only equal to the volume of water that a unit volume of saturated rock or soil will retain against gravity, which is always less than or equal to the total volume of voids in the rock or soil.