Answer:
Laziness.
Step-by-step explanation:
Rex had not even started his part of the project and his indolence (laziness) had the rest of his group worried
The sentence talks of a project that needed to be done but hasn't been. The rest of the group members were even worried. So, this could only mean that the project hasn't been finished. 'Indolence' means the 'reluctance or avoiding of doing any work that is needed to be done'. Accordingly, the use of indolence in the sentence will mean that Rex is lazy.