There are many sources of potential candidates

A but the best is to either hire from within the organization somebody that an existing employee has recommended

B and careful consideration of both internal and external sources must be considered. The right mix depends of the circumstances and the particular position to be filled

C and careful consideration of both internal and external sources must be considered. The right mix is $80\%$ external and $2020$ internal $-$ apply the $\frac{80}{20}$ rule.

D but in most cases it is best to use employment agencies as they have the expertise and extensive databases