The main organic compound obtained when ethylene glycol is heated with acidified potassium permanganate is formic acid (option C). Let's break down the process and explain why this is the correct answer.

1. Oxidation of ethylene glycol:
When ethylene glycol (HOCH2CH2OH) is heated with acidified potassium permanganate (KMnO4), it undergoes oxidation. The oxidation state of carbon in ethylene glycol is +2, and it is oxidized to an oxidation state of +4.

2. Formation of an aldehyde:
During the oxidation process, one of the hydroxyl groups in ethylene glycol is oxidized to an aldehyde functional group (CHO). This aldehyde group is then further oxidized to formic acid (HCOOH).

3. Oxidation of aldehyde to formic acid:
The aldehyde group is further oxidized by potassium permanganate to form formic acid. The oxidation state of carbon in the aldehyde group is +1, and it is oxidized to an oxidation state of +2 in formic acid.

4. Overall reaction:
The overall reaction can be represented as follows:
HOCH2CH2OH + 2[O] → HCOOH + H2O

5. Explanation of other options:
a) Oxalic acid (option A) is not the main product because it requires further oxidation of formic acid, and the reaction conditions described do not favor the formation of oxalic acid.
b) Glyoxal (option B) is not the main product because it is formed through the oxidation of ethylene glycol to an aldehyde, followed by further oxidation to a carboxylic acid. However, the reaction conditions described do not favor the formation of glyoxal.
d) Ethanol (option D) is not the main product because it is not formed through the oxidation of ethylene glycol. Ethanol can be obtained by reducing ethylene glycol, not oxidizing it.

In conclusion, when ethylene glycol is heated with acidified potassium permanganate, the main organic compound obtained is formic acid (option C).