Isostructural compounds are those that have the same shape and similar bond angles. To determine which compound is isostructural with carbon dioxide (CO₂), we need to analyze the molecular geometry of each of the options.

1. CO₂ has a linear structure with the carbon atom in the center, double-bonded to two oxygen atoms, giving it a bond angle of 180°. The structure can be represented as:

$$ \text{O} = \text{C} = \text{O} $$

2. HgCl₂ also has a linear structure. The mercury atom in the center is single-bonded to two chlorine atoms, giving it a bond angle of 180°. The structure can be represented as:

$$ \text{Cl} - \text{Hg} - \text{Cl} $$

3. SnCl₂ has a bent (or V-shaped) structure due to lone pairs on the tin atom. Hence, it is not isostructural with CO₂.

4. C₂H₂ (acetylene) has a linear structure with a carbon-carbon triple bond and hydrogen atoms at each end, giving it a bond angle of 180°. The structure can be represented as:

$$ \text{H} - \text{C} \equiv \text{C} - \text{H} $$

5. NO₂ has a bent structure due to the presence of a lone electron on the nitrogen atom, which causes repulsion and bends the molecule. Therefore, it is not isostructural with CO₂.

Given this analysis, CO₂ is isostructural with both HgCl₂ and C₂H₂.