(a) a+b=b+a,∀a,b∈N. So, addition is commutative in N. (b) (a×b)×c=a×(b×c),∀a,b,c∈N. So, multiplication is associative in N. (c) a*b=ab and b*a=ba ∴a*b≠b*a;∀a,b∈N So, * is commutative in N is false. (d) (a+b)+c=a+(b+c),∀a,b and c∈N. So, addition is associative in N.