tl;dr: ROUND.
Supporting argument:
Here's the pertinent text:
"3.E.1.a Rollbar Tubing and Spreader-Plate Specs. Minimum tubing size for cars weighing under 3000 pounds as raced is 1.50″ x .120″ or 1.75″ x .095″. Cars weighing over 3000 pounds as raced must use a minimum tubing size of 1.75″ x.120″. Properly bent, racecar-grade and -quality tubing is mandatory: no stretched or crushed bends allowed."
First, let's assume an understanding that as different vehicles have different dimensions, there no need to specify a length of tubing for any particular structural member.
Second, I agree with TheEngineer there's no "round"; but also no instance of the word "diameter" above.
And, OP did use "diameter" in his post.
But it does say "tubing", and there are two dimensions.
Reducto ad absurdum:
OP _could_ have made an inquiry about using flat stock for a cage, since there were only two dimensions;
for example: 1.75″ x.120″ - or: 1 3/4" wide, by about 1/8" thick.
... with the exception of the word, "tubing".
"Tubing" implies a hole - pretty much by definition.
"Diameter" relates to something circular.
All the square stock "for sale" listings I've seen discuss width x height x wall thickness, and then, length (which we're disregarding for this discussion).
Without a _third_ dimension specified, the "tubing size" specified actually _implies_ a diameter; again, by definition.
And since square stock at any size varies from a 'diameter' (and it's especially obvious at 1.5" or "1.75"), I'm going to contend the rules actually specify round tubing by definition and implication. Q.E.D.