To determine which of the responses shows that polynomials form a closed system under multiplication, we need to look for a product that results in another polynomial.
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\(5\cdot \frac {3}{x}\)
This expression simplifies to \(\frac{15}{x}\), which is not a polynomial (it has a negative exponent).
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\((\frac {1}{3})(\frac {x}{4}-2)\)
This expression simplifies to \(\frac{1}{3} \cdot \frac{x}{4} - \frac{2}{3} = \frac{x}{12} - \frac{2}{3}\), which is a polynomial.
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\(2x^{3}+x^{2}\)
This expression is already a polynomial.
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\(5x^{-1}\cdot 2\)
This simplifies to \(\frac{10}{x}\), which is not a polynomial (again has a negative exponent).
Among the given options, \((\frac {1}{3})(\frac {x}{4}-2)\) and \(2x^{3}+x^{2}\) both yield a polynomial. However, to show that polynomials form a closed system under multiplication, we specifically look to ensure that the multiplication of polynomials results in another polynomial.
Thus, while \((\frac {1}{3})(\frac {x}{4}-2)\) is a polynomial, what we want to establish closure is policy agreement on both capital \(2x^{3}+x^{2}\), which already is a polynomial. Although the only option indicating that upon multiplication the final result remains a polynomial would point towards, \(2x^{3}+x^{2}\) specifically does not denote a product.
Conclusively, none of options represent a multiplication result clearly demonstrating polynomial closure. The only clear polynomial is option \(2x^{3}+x^{2}\).
In terms of closure under multiplication with polynomials, we should choose:
\(2x^{3}+x^{2}\)
to express that the expression is already polynomial confirming closure basis.
However since option defines what denotes closure indicate represents stating we observe closure under the multiplication strictly whenever \(a, b \in R[x]\) thus:
\(\boxed{2x^{3}+x^{2}}\) and proportional representation conveys polynomial closure under multiplication rightfully.