A – Wrong
The discovery of other tools does not necessarult challenge the theory. These other tools could have been made by the Clovis after they came to America.
B – Wrong
Even if the Ice Age had lasted longer, it does not necessarily mean that people from Asia could not have entered the Americas.
C – Correct
If it was found that there were human settlements dating to 12,000 BC, this would mean that there were humans in America 14,000 years ago. This means that there were humans in America before the Clovis moved there 13,500 years ago. Hence, the Clovis would not have been the first inhabitants of the Americas.
D – Wrong
Extinction of the big mammals has very little correlation with the Clovis moving in to the Americas. This does not seriously challenge the theory.
E – Wrong
If this were true, it would have been easier for the Clovis to reach America from Asia.