Concept:In the compound noun "head of state", the main noun changes to its plural form when referring to more than one person.
Explanation:The singular form is "a head of state".
To make it plural, the principal noun "head" becomes "heads", while "of state" remains unchanged.
Thus, the correct plural is "heads of state".
The sentence refers to "all the African ...", which requires the plural form.
Therefore, the best completion is "heads of state".
Answer:B. heads of state