The correct option is B 6y is not a monomial
Definition of a Monomial:–––––––––––––––––––––––––––––––
Monomial is a type of expression which has only a single non-zero term in it. It consists of different parts like the coefficient, the variable, and its degree.
Identification of a Monomial:–––––––––––––––––––––––––––––––––––
A monomial expression must have a single non-zero term. The exponents of the term must be a non-zero whole number.
We are given with an expression 6y. It contains a variable exponent y. It is not a monomial.
Therefore, option (b.) is the correct answer.