Step 1: Find sink temperature
\(\eta = 1 - \frac{T_2}{T_1}\), so \(0.5 = 1 - \frac{T_2}{600}\) gives \(T_2 = 300\ \text{K}\).
Step 2: New efficiency
The efficiency increases by 20 percentage points, from \(50\%\) to \(70\%\).
Step 3: New source temperature
\(0.7 = 1 - \frac{300}{T_1'}\) gives \(\frac{300}{T_1'} = 0.3\), so \(T_1' = 1000\ \text{K}\). Option (C).
Final Answer:
The new source temperature is 1000 K.
\[ \boxed{\text{(C)}\ 1000\ \text{K}} \]