Step 1: Count electron regions:
Draw the Lewis structure of each molecule and count the regions (bonds and lone pairs) around the central atom.
Step 2: Match region count to shape:
Two regions with no lone pair give a line. Three regions give a flat triangle or a bent shape. Four regions give a pyramid or a tetrahedron.
$\text{CO}_2$: 2 regions, linear. $\text{SO}_2$: 3 regions (2 bonds plus 1 lone pair), bent. $\text{BCl}_3$: 3 regions, trigonal planar. $\text{NH}_3$: 4 regions (3 bonds plus 1 lone pair), pyramidal.
Step 3: Conclusion:
Only $\text{CO}_2$ is a straight, linear molecule.
Final Answer:
The linear molecule is $\text{CO}_2$, option (C).
\[ \boxed{\text{CO}_2 \text{ (C)}} \]