[tex]f(x)=3x+1\\g(x)=5x-1[/tex]
[tex]\dfrac{f}{g}(x)[/tex]
The domain of (f/g)(x).
Substitute for f(x) and g(x) in the desired relatinon.
[tex]\dfrac{f}{g}(x)=\dfrac{f(x)}{g(x)}=\dfrac{3x+1}{5x-1}[/tex]
The domain must exclude the value of x that makes the denominator equal to zero.
[tex]5x-1=0\\5x=1\qquad\text{add 1}\\x=\frac{1}{5}\qquad\text{divide by 5}[/tex]
The domain must exclude x=1/5.
This matches the last choice.