First step is to get rid of the square root. remember that a square root is the same as raising to the (1/2) power. see below:
 } } }{ { \sqrt{50} x}^ {{36}^{ (\frac{1}{2}) } }})
multiply exponents per exponent rules to get:

remember you subtract exponents from the denominator and add exponents in the numerator so:
 } ) = \sqrt{ \frac{36}{25} } \times ( {x}^{ - 10} ))
simplify further:

so a is the correct choice. if you don't understand this google exponent rules and it will you understand. look at the image results.