Which of the following is a binomial?

1. a² - 6b² + 1000
2. -7x²y + xy² + 3x²y
3. ma²b - (m + n)a²b² + nab²
4. st² - st + 1

A binomial must have exactly two terms after simplification.

Analysis:

1. a² - 6b² + 1000

There are no like terms to combine. It has 3 terms.

2. -7x²y + xy² + 3x²y

Let's simplify:

-7x²y + 3x²y + xy² $\quad$[Grouping like terms]

= (-7 + 3)x²y + xy²

= -4x²y + xy²

It has 2 terms. It is a binomial.

3. ma²b - (m + n)a²b² + nab²

There are no like terms to combine. It has 3 terms.

4. st² - st + 1

There are no like terms to combine. It has 3 terms.

Hence, option 2 is the correct option.