Chlorofluorocarbons (CFC's) pose known dangers to public health. Only

B is the correct one.

Chlorofluorocarbons (CFC's) pose known dangers to public health. Only when the United States government imposes a specific ban on the industrial use of CFC's will industry scientists make the alternatives to CFC's cost-effective, and thus reduce public health hazards.

Which of the following is an assumption on which the assertion made above is based?

(A) The alternatives to CFC's currently available are not widely used because they are not familiar to a sufficient number of industry scientists. - the alternatives will be in place in future when scientists start researching CFC's alternatives after the Govt pose a ban on it.
(B) The alternatives to CFC's are less hazardous to public health than are CFC's. - Now this option is correct; Negating the option clearly shatters the conclusion as if the alternatives are more hazardous to public health than there is no way it will reduce health hazards;
(C) Private industry has a responsibility to take voluntary measures to safeguard public health and absorb the costs of such measures - who cares what the responsibility of private industries & borne the cost of safeguarding public health, nothing to do with the argument
(D) The use of CFC's can result in employee time lost because of illness. - if employee time lost because of illness was an effect of CFC's usage then the industry scientists will voluntarily try to figure out alternatives, not to wait for directives from the GOVT.
(E) CFC's are currently the most serious public health hazard engendered by industry in the United States - too strong a phrase "the most serious public health hazard" to be assumed.