The efficiency of a Carnot engine is given by the formula:
\[
\eta = 1 - \frac{T_{\text{sink}}}{T_{\text{source}}}
\]
Where:
- \(\eta\) is the efficiency,
- \(T_{\text{sink}}\) is the temperature of the sink (cold reservoir),
- \(T_{\text{source}}\) is the temperature of the source (hot reservoir).
We are given that the efficiency \(\eta = 0.40\), so:
\[
0.40 = 1 - \frac{T_{\text{sink}}}{T_{\text{source}}}
\]
Solving for the ratio \(\frac{T_{\text{sink}}}{T_{\text{source}}}\):
\[
\frac{T_{\text{sink}}}{T_{\text{source}}} = 1 - 0.40 = 0.60
\]
Thus, the correct answer is (B) 0.6.