Instead of tracking old and new income in separate stages, let's build a clean picture of the two conversions the person makes and read off the answer from the totals.
He starts with Rs. 10,000 in 4% debentures, which pays him a fixed Rs. 400 every year (that is simply $4\%$ of $10000$). On 1st Jan 1970 he splits this holding into two parts and swaps both parts into new instruments.
Now add up the two new income streams: $240 + 175 = 415$. This Rs. 415 replaces the old Rs. 400 he was earning from the debentures.
Comparing the two: $415 - 400 = 15$. Since the new figure is larger, his income has gone up, not down, by Rs. 15.
Let's summarize:
So the correct answer is a Rs. 15 increase in his annual income, matching option (3).
\[ \boxed{\text{Rs. 15 increase}} \]