Upvote if you didn't know the answer and had to Google it? Downvote if everybody knows the answer and it's a stupid question?
To save people googling, it appears that I was right [0], however this is not try for all greenhouse gasses. For instance, methan grows as sqrt(x), and CFCs are linear.
Admittedly, the linked expressions are empirical, so only represent an approximation for the range of values that we have observed. There are also dynamic effects (such as increased water vapor, or a runaway release of trapped methane) that are not being moddeled by estimates of thermal forcing.
All questions about the legitimacy of climate science are questions about whether simulations of the earth's atmosphere ("general circulation models") can be validated or have predictive power, at least with realistic computing resources.
However, the big-O question is an interesting reflection on the ratio between how much scientific communication effort goes into influencing the public, versus educating the public.
There is also not much funding available for studying positive impacts of global warming. Scientists have known about the Arrhenius effect for over a century, and in pre-WWII texts the treatment is generally "isn't it cool that burning fossil fuels not only gives us power and light, but is good for agriculture and might prevent the next ice age?" The past is a foreign country, and in that country it was generally assumed that warm periods were good for humans and cold ones were bad. (Mostly because ag yields.) They didn't even really think about it -- and nor do we think much about it, either. Plus ca change...
(A) is false - while water vapour has high radiative forcing, it is not "an important greenhouse gas" in the big picture, unlike what the coal industry reps say. This [1] should explain why. Methane and CFCs are far less important in aggregate (about 1/3 and 1/4 of the effect of CO2 respectively), although they are far more dangerous per unit of volume.
(C) is obviously false - even in the low level layman explanation in [1] you can see that they include a table with the forcings of various combinations (like H2O vapour+CO2); you can also see that the table is from 1978, so scientists have done that since way before you were born.
(D) false
[1] http://www.realclimate.org/index.php/archives/2005/04/water-...
Good question to pose.
I'm also upvoting because that "sounds" like maybe you are pessimistic about the significance and need them to recoup the robo downvotes given to anyone not willing to swear the worlds going to end tomorrow. (don't know for sure if they lurk on HN, but for sure they are everywhere else).
for some details