Explanation:

Given Molecular Formula: C3H9N

Step 1: Determining the number of carbon isomers:
- Since the molecular formula contains 3 carbon atoms, we can have different arrangements of carbon atoms in the structure.
- The possible carbon chain isomers are: propane (3 carbons in a straight chain) and methylpropane (2 carbons in a chain with a methyl group).
- Therefore, there are 2 carbon chain isomers possible.

Step 2: Determining the number of hydrogen isomers:
- The molecular formula C3H9N determines the number of hydrogen atoms in the molecule.
- The possible hydrogen isomers are determined by the different ways hydrogen atoms can be arranged on the carbon chain.
- For propane, there are 9 hydrogen atoms in a straight chain.
- For methylpropane, there are 9 hydrogen atoms with a methyl group attached to one of the carbons.
- Therefore, there are 2 hydrogen isomers possible.

Step 3: Combining carbon and hydrogen isomers:
- By combining the carbon chain isomers with the hydrogen isomers, we get the following structural isomers:
1. Propane (3 carbons in a straight chain)
2. Dimethylmethane (2 carbons in a chain with a methyl group)
3. Dimethylamine (2 carbons in a chain with a methyl group and an amino group)
4. Ethylamine (3 carbons in a straight chain with an amino group)
- Therefore, there are 4 structural isomers possible from the molecular formula C3H9N.
Therefore, the correct answer is option 'D' (4).