At the end of the 20th century and at the start of the 21st century, the united states has been providing large amounts of humanitarian support to countries in africa. which of these is a reason for most of that support?

I don't know what your options were, but here are some reasons for the US to provide support to countries in Africa:

-moral reason: it's morally right to share the wealth
-humanitarian reasons: we care about other people and we don't want other people to suffer
-economic reasons: if the US provides aid to those countries, it can later invest in them too, which will be economically profitable.
-it's also generally good if other countries are also rich, as they can be a potential market for the US goods