Concept:
Irrigation efficiency describes how effectively water is conveyed, applied, and distributed in the field.
Step 1: Understand conveyance efficiency.
Water conveyance efficiency compares water delivered to the field with water diverted from the source.
\[
E_c=\frac{\text{Water delivered to field}}{\text{Water diverted from source}}\times100
\]
Step 2: Identify the given definition.
The question states the ratio of water reaching the farm or field to the water diverted from the irrigation source.
This exactly represents:
\[
\text{Water conveyance efficiency}
\]
Step 3: Final answer.
\[
\therefore \text{Correct Answer is (B)}
\]