Given general formula

We need to identify which open-chain compound class can have the molecular formula $C_nH_{2n}O_2.$

Check each option one by one

Option A: Carboxylic acids

The general formula of a saturated open-chain monocarboxylic acid is $RCOOH,$ where $R = C_{n-1}H_{2(n-1)+1} = C_{n-1}H_{2n-1}$.

So, $RCOOH = C_nH_{2n}O_2.$

Example: acetic acid $CH_3COOH = C_2H_4O_2$.

Therefore, Option A fits.

Option B: Diols

A saturated open-chain diol has general formula $C_nH_{2n+2}O_2.$

Example: ethylene glycol $C_2H_6O_2$.

This is not equal to $C_nH_{2n}O_2$.

Therefore, Option B does not fit.

Option C: Dialdehydes

An open-chain dialdehyde has two terminal $-CHO$ groups.

Start from alkane formula: $C_nH_{2n+2}$

Converting two terminal $-CH_3$ groups into two $-CHO$ groups removes $4$ H and adds $2$ O: $C_nH_{2n+2} \to C_nH_{2n-2}O_2.$

Example: glyoxal is $C_2H_2O_2$.

So dialdehydes have formula $C_nH_{2n-2}O_2,$ not $C_nH_{2n}O_2$.

Therefore, Option C does not fit.

Option D: Deketones

An open-chain diketone has two ketone groups.

Compared to alkane $C_nH_{2n+2}$, introduction of two ketone groups removes $4$ H and adds $2$ O: $C_nH_{2n+2} \to C_nH_{2n-2}O_2.$

Example: butane-2,3-dione is $C_4H_6O_2$.

So deketones also have formula $C_nH_{2n-2}O_2.$

Therefore, Option D does not fit.

Conclusion

The only open-chain class matching $C_nH_{2n}O_2$ is carboxylic acids.

Comparison with stored correct answer

Stored correct answer: A

Derived answer: A

Hence, the derived answer agrees with the stored correct answer.