Answer :
the answer is D. ax4y6
proof
the binome of newton is S a x^k y^n-k = (x+y)^n, for k=0...n
for n=10, it is a y^10+...+ a x4 y^10-4 +...+ax^10
proof
the binome of newton is S a x^k y^n-k = (x+y)^n, for k=0...n
for n=10, it is a y^10+...+ a x4 y^10-4 +...+ax^10