Step 1: Concept
Compare the side ratios with the standard side ratios of a $30^\circ$-$60^\circ$-$90^\circ$ triangle.
Step 2: Meaning
The given ratio is
\[
1:\sqrt3:2.
\]
This is exactly the side ratio of a right triangle with angles
\[
30^\circ,\;60^\circ,\;90^\circ.
\]
Step 3: Analysis
The side opposite $30^\circ$ is proportional to $1$.
The side opposite $60^\circ$ is proportional to $\sqrt3$.
The side opposite $90^\circ$ is proportional to $2$.
Hence the angles are
\[
30^\circ:60^\circ:90^\circ.
\]
Dividing by $30^\circ$,
\[
1:2:3.
\]
Step 4: Conclusion
Therefore the required ratio is
\[
1:2:3.
\]
Final Answer: (A)