Concept:
The efficiency of a Carnot engine is
\[
\eta
=
1-\frac{T_2}{T_1},
\]
where
\[
T_1=\text{source temperature},
\qquad
T_2=\text{sink temperature}.
\]
Step 1: Find the source temperature.
Initially,
\[
\eta=50\%=0.5,
\qquad
T_2=500\,\text{K}.
\]
Using
\[
0.5
=
1-\frac{500}{T_1},
\]
\[
\frac{500}{T_1}
=
0.5.
\]
\[
T_1
=
1000\,\text{K}.
\]
Step 2: Calculate the new sink temperature for \(60\%\) efficiency.
Given,
\[
\eta=60\%=0.6.
\]
Using
\[
0.6
=
1-\frac{T_2'}{1000},
\]
\[
\frac{T_2'}{1000}
=
0.4.
\]
\[
T_2'
=
400\,\text{K}.
\]
Step 3: Write the final answer.
\[
\boxed{T_2'=400\,\text{K}}
\]
\[
\boxed{\text{Answer = (B)}}
\]