The expression "forty and two hundred seventy-four thousandths" can be broken down as follows:
- "Forty" refers to 40.
- "Two hundred seventy-four thousandths" refers to \( \frac{274}{1000} \), which is 0.274.
Combining these, we get:
\[ 40 + 0.274 = 40.274. \]
Thus, the correct answer is:
B. 40.274.