Formic acid is obtained when :

Options

A. calcium acetate is heated with conc. `H_2SO_4`
B. calcium formate is heated with calcium acetate
C. glycerol is heated with oxalic acid at 373 K
D. acetaldehyde is oxidised with `K_2Cr_2O_7` and `H_2SO_4`

Explanation

To solve the question "Formic acid is obtained when:", we need to analyze each of the provided options to determine which reaction leads to the formation of formic acid.

### Step-by-Step Solution:

1. **Analyze Option A: Calcium acetate is heated with concentrated H2SO4.**
- When calcium acetate is heated with concentrated sulfuric acid, it typically leads to the formation of acetic acid and other by-products, but not formic acid. Therefore, this option does not yield formic acid.

**Hint:** Consider the products formed when heating calcium acetate with sulfuric acid.

2. **Analyze Option B: Calcium formate is heated with calcium acetate.**
- Heating calcium formate with calcium acetate generally results in the formation of carbonyl compounds, not formic acid. Thus, this option is also incorrect.

**Hint:** Think about the types of reactions that occur when calcium salts are heated together.

3. **Analyze Option C: Glycerol is heated with oxalic acid at 373 Kelvin.**
- When glycerol (C3H8O3) is heated with oxalic acid (C2H2O4) at 373 Kelvin, a series of reactions occur:
- First, glycerol reacts with oxalic acid to form glycerol mono-oxalate.
- Then, through dehydration (removal of water) and decarboxylation (removal of CO2), glycerol mono-oxalate converts to glycerol monoformate.
- Finally, hydrolysis of glycerol monoformate yields glycerol and formic acid (HCOOH).
- Therefore, this option is correct as it leads to the formation of formic acid.

**Hint:** Focus on the specific conditions and reactants that lead to the formation of formic acid.

4. **Analyze Option D: Estraldehyde is oxidized with K2Cr2O7 and H2SO4.**
- The oxidation of acetaldehyde (which seems to be a typo for "estraldehyde") with potassium dichromate (K2Cr2O7) in acidic conditions typically produces acetic acid, not formic acid. Thus, this option is incorrect.

**Hint:** Recall the products of oxidation reactions involving aldehydes.

### Conclusion:
The correct answer is **Option C: Glycerol is heated with oxalic acid at 373 Kelvin**, which leads to the formation of formic acid.