Pi does not equal 4, even in certain topologies. Pi=3.14159... in any topology. It may be that the ratio of the circumference of a circle to its diameter is 4 under a particular metric (and using some obvious definition of circle), but in that case it is not pi!
Furthermore, some of the above suggested violations of possibility seem more grave than others because presumably they involve different sorts of possibility. It's not possible for me to fly to Pluto in a week, but this is a practical impossibility, rather than a physical or logical impossibility. Similarly, it's a physical rather than logical impossibility that precludes any massed object travelling at light speed and so it seems odd to invoke ex falso quodlibet, a principle of logic, for a question framed in the form of a counterfactual about physical laws. I think it is comparably odd to a claim of the form 'if it were the case that I could fly to Pluto in a week, one would equal zero'. I'm confident that the question-setter here doesn't intend any logical inconsistency in his/her question and would gladly concede that if it were the case that an object (with mass) could travel at light speed, it would not be the case that nothing (with mass) could travel at light speed.
Perhaps the question would be better phrased 'If an object with no mass travelled at the speed of light, would it be invisible?' Indeed some particles are massless and do travel at the speed of light - in particular the photon! And photons certainly aren't invisible (at least at some level of detection).
Now suppose some massless object A is travelling at velocity c relative to some observer O and it emits a photon along a geodesic in the direction AO it would velocity c relative to A and to O! Similarly if O emits a photon along a geodesic in the direction OA, the photon would have velocity c relative to both A and O. Hence, with appropriate light sources, A need not be invisible to O!