If ${\left(\frac{1+i}{1-i}\right)}^x=1$, then:

1. A
   $x=4n$, where $n$ is any positive integer
2. B
   $x=2n$, where $n$ is any positive integer
3. C
   $x=4n+1$, where $n$ is any positive integer
4. D
   $x=2n+1$, where $n$ is any positive integer