Which of the following expression is monomial?

Let us check each option one by one.

Option A: 4y − 7z

Since the given expression contains two terms, it is a binomial.

Option B: y²

Since the given expression contains only one term, it is a monomial.

Option C: x + y − xy

Since the given expression contains three terms, it is a trinomial.

Option D: ab − a − b

Since the given expression contains three terms, it is a trinomial.

Hence, the expression y² is the correct alternative.