Identify substance K based on its reactions with concentrated sulfuric acid, aqueous silver nitrate and nitric acid, and aqueous ammonia.

To identify substance K, we analyze the provided observations and inferences:

1. Reaction with concentrated sulfuric acid:

   • Observation: "Turns blue litmus paper red and has no effect on red litmus paper." (This observation is typically for gases, but the inference directly identifies K).
   • Inference: "Oxalate or oxalic acid (solid) present. Dehydration $\text{CO}_2$ and $\text{CO}$ gases evolved. $\text{COOH}_2 + \text{H}_2\text{SO}_4 \to \text{CO} + \text{CO}_2 + \text{H}_2\text{O} + \text{H}_2\text{SO}_4$."
   • This inference strongly indicates that K is oxalic acid ($\text{H}_2\text{C}_2\text{O}_4$ or $\text{COOH}_2$). The reaction shown is the characteristic dehydration of oxalic acid by concentrated sulfuric acid.

2. Reaction with aqueous silver nitrate and nitric acid (Test iii):

   • Observation: "White precipitate when $\text{AgNO}_3$ and acid is added."
   • This is consistent with oxalic acid forming silver oxalate ($\text{Ag}_2\text{C}_2\text{O}_4$), which is a white precipitate.

3. Reaction with aqueous ammonia (Test iv):

   • Observation: "White precipitate dissolves and becomes clearer upon addition of aqueous $\text{NH}_3$."
   • Silver oxalate ($\text{Ag}_2\text{C}_2\text{O}_4$) is known to dissolve in aqueous ammonia due to the formation of the diamminesilver(I) complex, $[\text{Ag}(\text{NH}_3)_2]^+$. While the inference provided for this step is not directly relevant to oxalates, the observation itself supports the presence of silver oxalate.

Based on the strong evidence from the reaction with concentrated sulfuric acid and the supporting observations with silver nitrate and ammonia, K is oxalic acid.

$$\boxed{\text{Oxalic acid}}$$