Hi!
The most important factor for a successful survey is
C. Proper sample size
Sample size contributes the statistical significance of the data acquired. A sample study that has an inadequate and low sample size will yield inconclusive results and not support the survey. The results would not be justifiable and the margin of error will be sufficiently less. Anomalies that may arise will be more profound and could change the face of the study entirely.
A sample size that would be too large would result in waste of precious resources and time.
Hope this helps.