Step 1: Weighted-mean idea:
Think of 100 chlorine atoms. Of these, 75 have mass number 35 and 25 have mass number 37. The average atomic mass is the total mass of the 100 atoms divided by 100.
Step 2: Total mass of the sample:
Mass of 75 light atoms is $75 \times 35 = 2625$ u.
Mass of 25 heavy atoms is $25 \times 37 = 925$ u.
Total mass is $2625 + 925 = 3550$ u.
Step 3: Average per atom:
\[ \frac{3550}{100} = 35.5 \text{ u} \]
A shortcut check: start at 35 and add the heavier isotope's extra 2 u times its fraction, $35 + 2 \times 0.25 = 35.5$.
Step 4: Check the options:
The value must lie between 35 and 37. Options 35.0 and 37.0 are the two isotopic masses themselves, and 37.5 is out of range, so only 35.5 fits.
Final Answer:
The average atomic mass is 35.5 u, so option (B) is correct.
\[ \boxed{35.5} \]