To determine which structures are chiral, we check whether each molecule is non-superimposable on its mirror image. A quick practical test is:

1. Look for a stereogenic center (usually an $sp^3$ carbon attached to four different groups).
2. Also check for internal symmetry (plane of symmetry / center of symmetry), because a molecule with such symmetry can be achiral even if it appears substituted.

Since the image of the four structures is not shown here, the conclusion must be inferred from the given options and stored answer. The stored correct answer says:

$\text{Only } i \text{ and } ii \text{ are chiral compounds.}$

So the correct evaluation is:

• Structure $i$: chiral
• Structure $ii$: chiral
• Structure $iii$: achiral
• Structure $iv$: achiral

Thus, the true statement is:

$\boxed{\text{Only } i \text{ and } ii \text{ are chiral compounds}}$

Hence, Option A is correct.

Option-wise check

• A: only $i$ and $ii$ are chiral compounds — Correct
• B: only $iii$ is a chiral compound — Incorrect
• C: only $ii$ and $iv$ are chiral compounds — Incorrect
• D: all four are chiral compounds — Incorrect

Final Answer

$\boxed{A}$