An organic compound L is known to contain only C, H, and O. It gives a luminous sooty flame. When heated with aqueous NH3, it gives off a gas which turns limewater milky. It is also found that the par

Based on the observations and inferences:

1. The "luminous sooty flame" indicates a high carbon-to-hydrogen ratio, characteristic of a highly unsaturated or aromatic compound.
2. The inference "Product is an amide" from the reaction with aqueous $\text{NH}_3$ clearly identifies the functional group as an amide ($\text{CONH}_2$).
3. The inference "Substance is an amide. Parent is stronger than carbonic acid $\text{CO}_2$ evolved" further confirms it is an amide and that the carboxylic acid from which it is derived is strong enough to react with carbonates. Benzoic acid is an example of such a parent acid.

Combining these, the simplest possible structural form for L is an aromatic amide. The simplest aromatic group is the phenyl group ($\text{C}_6\text{H}_5$).

Therefore, the simplest possible structural formula for L is benzamide:

$$\boxed{\text{C}_6\text{H}_5\text{CONH}_2}$$