What is the greatest limitation that scientists are dealing with as they try to figure out how to experiment with the ebola vaccine?
a. time
b. complexity
c. money
d. ethics

Iagree with  pbjgurl123 but would suggest that you also consider the implications of each answer. they are all technically correct.
a is correct simply due to the fact that the longer time passes without an ebola cure, the more people can be potentially infected with it, resulting in  less healthy individuals that may be able to volunteer to test potential ebola vaccines. scientists need healthy volunteers from affected regions in order for rapid clinical trials to occur.
b is correct because of the nature of epidemics. a combination of technical, social, economic, and geographical obstacles slow the progress and dissemination  of ebola vaccine information.
c is correct because finance impacts everything from finding resources used in studying ebola  to distributing vaccines to affected areas. economic imbalances or money shortages negatively impact research.
d is correct due to many social factors (more of a subjective answer). some people think that vaccine distributors discriminate against certain ethnic groups. others think the governments/other organizations are not effective enough at managing the situation. still others feel that the search for a cure is not being approached in the right way. in all, there are multiple ways to oppose different aspects of ebola research.
ultimately, b is correct because it seems to capture the meaning of a, c, and d together. 

B. is the answer glad 2 u