Explanation:

- Let's consider Mohan's initial income as $100.

- In the first month, his income increases by 30%. So, 30% of $100 is $30. His new income is $100 + $30 = $130.

- In the second month, his income decreases by 30%. So, 30% of $130 is $39. His new income is $130 - $39 = $91.

- Comparing the final income with the initial income: $91 is $9 less than $100.

- Therefore, the overall change is a decrease of $9 from the initial $100.

- This results in a 9% decrease.

Correct Answer is: Option:4 - 9% decrease