Which statement is true?

Answer: (d)
Step-by-step explanation:
For option (a)
[tex]\sqrt[m]{x}\sqrt[m]{y}=\sqrt[m]{xy}[/tex]
for the same exponent, numbers multiply keeping exponent the same.
for option (b)
[tex]a\sqrt[n]{x} +b\sqrt[n]{y} =\left(a+b\right)\sqrt[n]{x}[/tex]
for option (c)
[tex]\Rightarrow \dfrac{\sqrt[m]{x} }{\sqrt[m]{y} }=\sqrt[m]{\dfrac{x}{y}}[/tex]
for option (d)
exponent multiplies
[tex]\therefore \left(\sqrt[m]{x^a} \right)^b=\sqrt[m]{x^{ab}}[/tex]
option (d) is correct