Which of the following responses shows that polynomials form a closed system under multiplication?

To determine if polynomials form a closed system under multiplication, we need to check if the result of multiplying any two polynomials results in another polynomial.

Let's analyze the expressions you've provided:

1. \(5x - 1.2\) is a polynomial of degree 1 (linear).
2. \(3\) is a constant polynomial (degree 0).
3. \(7 - 2\) simplifies to \(5\), which is also a constant polynomial (degree 0).
4. \(5\) is another constant polynomial (degree 0).
5. \(2x^3 + x^2\) is a polynomial of degree 3.

To check for closure under multiplication, let's perform some multiplications:

- **Multiplying \(5x - 1.2\) and \(2x^3 + x^2\)**:
\[
(5x - 1.2)(2x^3 + x^2) = 10x^4 + 5x^3 - 2.4x^3 - 1.2x^2 = 10x^4 + (5 - 2.4)x^3 - 1.2x^2
\]
This result is clearly a polynomial (degree 4).

- **Multiplying \(3\) and \(2x^3 + x^2\)**:
\[
3(2x^3 + x^2) = 6x^3 + 3x^2
\]
This result is also a polynomial (degree 3).

- **Multiplying \(5\) and \(3\)**:
\[
5 \cdot 3 = 15
\]
This is a constant polynomial (degree 0).

Based on these examples, we see that multiplying two polynomials gives another polynomial each time.

Thus, polynomials are closed under multiplication.

If you are looking for a response that confirms this, consider stating:

"Polynomials form a closed system under multiplication because the result of multiplying two polynomials is also a polynomial."

However, if the options in your question represent responses concerning this closure property, please clarify or provide the possible answers more clearly for further analysis.