Just use Induction and an elementary identity.
Clearly:
\[
\binom{n}{0}=1=\binom{n}{n}\; ,
\]
Hence \(\binom{n}{0}\) and \(\binom{n}{n}\) are positive integers for any \(n\).
Therefore, it suffices to prove that:
For any \(n\geq 2\), all the binomial coefficients \(\binom{n}{k}\) with lower index \(k=1,\ldots ,n-1\) are integers.
Proof: For \(n=2\) the claim is true by inspection.
Now assume that the claim is true for \(n\) and prove it holds for \(n+1\).
The binomial coefficents satisfy the identity:
\[
\forall k\in \{ 1,\ldots ,n\},\quad \binom{n+1}{k}=\binom{n}{k-1}+\binom{n}{k}\; ;
\]
thus all the coefficients \(\binom{n+1}{k}\) with \(k=1,\ldots ,n\) are positive integers, for they are sum of the two positive integers \(\binom{n}{k-1},\ \binom{n}{k}\). \(\square\)