The correct answer is alternative B) "Muslim invaders conquered the country".
Muslims have never conquered Zimbabwe.
A northern kingdom (The United Kingdom) has already ruled Zimbabwe, which makes it a possibility.
There has been significant growth in Zimbabwe's population.
There have been droughts in Zimbabwe, as well as over farming, that destroys wild life, which is one of the main Tourism opportunities for the country. Less wild life leads to less tourism, which leads to less money in the country, being a possible cause for economic collapse.