Can both $f(n) \neq O(g(n))$ and $g(n) \neq O(f(n))$ happen at once?
Consider the following statement about two functions $f,g$:


  $f(n) \neq O(g(n))$ and $g(n) \neq O(f(n))$.


The question wants you to choose which of the following holds:


The statement holds for every $f,g$.
The statement holds for no $f,g$.
The statement holds for some $f,g$, and doesn't hold for other $f,g$.


The third option is correct in your case, and this is the meaning of "sometimes true".

Indeed, if $f(n) = g(n) = 1$ then the statement doesn't hold. Conversely, if $f(n) = 1$ and $g(n) = |n\sin n|$ (where $\left|\cdot\right|$ denotes absolute value) then the statement does hold.