7..........................

Answer: option c
Step-by-step explanation:
By definition, if you have:
[tex]\sqrt[n]{x}[/tex]
you can rewrite it has following:
[tex]x^{\frac{1}{n}}[/tex]
Therefore, keeping the above on mind, you can rewrite the expression given in the problem, as you can see below:
[tex]15x^{\frac{1}{3}}y^{\frac{1}{5}}=(15\sqrt[3]{x})(\sqrt[5]{y})[/tex]
Both terms are multiples of 15, then take the 15th root of both and multiply the exponents by 15. Therefore you obtain:
[tex]15\sqrt[15]{x^5y^3}[/tex]
The answer is:
c. [tex]15\sqrt[15]{x^{5}y^{3}}[/tex]
To express the expression using a radical we must remember that:
Transforming radical to exponential form:
[tex]\sqrt[n]{x}=x^{\frac{1}{n}}\\\sqrt{x}=x^{\frac{1}{2} }[/tex]
So, the given expression is:
[tex]15x^{\frac{1}{3}}y^{\frac{1}{5}}[/tex]
Its radical form will be:
[tex]15\sqrt[3]{x}\sqrt[5]{y}[/tex]
Then, the expression could be also equivalent to:
[tex]15\sqrt[3]{x}\sqrt[5]{y}=15\sqrt[15]{x^{5}}\sqrt[15]{y^{3}}\\\\15\sqrt[15]{x^{5}}\sqrt[15]{y^{3}}=15\sqrt[15]{x^{5}y^{3}}[/tex]
Have a nice day!