(A). The expression is , this expression is a product of variables, so it is a monomial.

(B). The expression is ,An expression that involves division by a variable, like , is not a monomial.

(C). The expression is ,this expression is a product of variables, so it is a monomial.

(D). The expression is ,this expression is a product of variables, so it is a monomial.

So, is not a monomial.

The option (B) is correct.