Which is equivalent to 16 3/4x?
-4√16^3x
-4√16^3
-3√16^4x
-3√16^4

The equivalent to 16^(3/4x) is  ⁴ˣ√16³.

It reads, 4x root of 16 raised to the power of 3.

16^(3/4x) can be written like this:  (16³)^1/4x  

1/4x as an exponent means the 4x  root of the base number. 
3 as an exponent simply means that the base number is raised to the third power.