The problem asks for the correct expression of average particle size with an index value of \( p = 1 \). The index \( p \) is related to the size of an individual particle, and the frequency index is given as \( f = 2 \). This suggests that we need to use the appropriate formula for particle size distribution.
The general formula for the size index is:
\[
d_{pq} = \frac{\sum n d^{p+q}}{\sum n d^{p+q-1}}
\]
For \( p = 1 \) and \( q = n \), we substitute these values:
\[
d_{s1} = \frac{\sum n d^2}{\sum n d^3}
\]
This matches option (B), making it the correct answer.