It is believed that increasing amounts of greenhouse gases in the atmosphere will result in
a. higher average global temperatures
b. cooler average global temperatures
c. no changes in average global temperatures

"Belief" is not involved.  

Choice-A is the correct answer. Except that it's not "... will result in ...".  It's already happening.

The people who have been keeping records of temperatures at a thousand places around the world, for the past hundred years, do not "believe".  They know that average global temperatures are increasing, and have been increasing steadily for a couple of Centuries now.

Working from hypotheses, models, observations, and proven laws of nature, the global scientific community are convinced that increasing amounts of greenhouse gases in the atmosphere, driven by human activity, are the cause of the trend in average global temperatures.

Yes, you can find published researchers who aren't so sure, and who disagree with this conclusion.  The scientific community is never 100% unanimous.  There were reasonable investigators and scientific observers who disagreed with Galileo, Newton, Faraday, Maxwell, Hawking and Einstein, in their time.  

But the voices that dispute human-caused climate change in the form of global warming are few, and they're becoming fewer as time goes on. Today, their reputation sits just a few notches above that of the Flat-Earthers, and it's continuing to sag further.

If we are to do what has to be done, in time to bring about the needed effect, it can't sag fast enough.

a coal burning power plant