A system of rotating crops with legume or grass pasture to improve soil structure and fertility is called as Ley farming, Strip farming, Contour farming, Shifting agriculture

Farming of legumes/grasses and other crops in alternation to improve the soil fertility is called as ley farming. Legumes add nitrogenous compounds to soil and thereby help the soil to resume the lost minerals. This makes option A correct.
The farming in row patterns that run nearly level around the hill is called as contour farming. Option B is incorrect.
Option C is incorrect as strip farming includes dividing the whole field in strips and cultivation in alternate strips to ensure soil fertility.
Option D is incorrect as shifting agriculture includes farming in one field and then leaving it for natural vegetation by cultivating in some other field.
Hence, correct answer is A.