If Africa were nuked, how would they be able to retaliate?

Let's say one of the nuclear superpowers for any reason believed that an African country posed a threat to its national defense, and as a result opted to nuke them. Since no African country has nuclear weapons, and thus no mutual ured destruction, what could they do about it? Since no other nuclear power that has signed to protect various African countries will be stupid enough to nuke the country, which means mutual ured destruction for them as well, how can they respond?