Concept:
Sizing is applied to warp yarn to improve strength, abrasion resistance, and weaving performance. The percentage of size add-on depends on fabric type and yarn quality. For cotton poplin, a medium level of sizing is required to balance strength and flexibility.
Step 1: Understand poplin requirement.
Poplin is a medium-weight fabric requiring moderate sizing.
Step 2: Relate sizing percentage.
Typical size add-on for cotton poplin:
\[
10\% - 12\%
\]
Step 3: Compare options.
This range matches moderate sizing requirement.
Step 4: Final answer selection.
\[
\boxed{10\% \text{ to } 12\%}
\]
Thus,
\[
\boxed{\text{Option (B) is correct.}}
\]