The question asks, "Which of these is a monomial?"

A monomial is an algebraic expression that consists of only one term. This term can be a constant, a variable, or a product of constants and variables raised to non-negative integer powers. It cannot include addition or subtraction.

Let's analyze the options:

1. 12xy – This is a monomial because it is a single term made up of the constant $12$ and the variables $x$ and $y$.
2. x² + y² – This is not a monomial because it consists of two terms.
3. 2x - 8 – This is not a monomial because it involves subtraction, so it has two terms.
4. x + y – This is not a monomial because it consists of two terms.

The correct answer is 12xy.

Would you like more details or have any questions?

Here are some related questions:

1. What is the definition of a polynomial?
2. How is a binomial different from a monomial?
3. Can a monomial include negative exponents?
4. How do you multiply two monomials?
5. Can a monomial include division between variables?

Tip: A monomial can contain constants and variables, but it must be a single term with no addition or subtraction.