Answer
$f(x) =\frac{1}{x} $
Work Step by Step
$(f ∘ g)(x) = \frac{1}{x^2+3}$
$g(x)=x^2+3$
$(f ∘ g)(x) = f(g(x)) = f(x^2+3) = \frac{1}{x^2+3}$
Therefore lets choose $f(x) =\frac{1}{x} $
$(f ∘ g)(x) = f(g(x)) = \frac{1}{g(x)} =\frac{1}{x^2+3}$