For some reason I cannot find my book, but from my notes, it seems like it's asking you for the capacitance between a and b. If you re-draw the circuit so that a and b are the points that you are looking into the circuit at, point c will just hang off the circuit and not have any influence on the capacitance.
Think of it as putting a voltage source between points a and b. Point c doesn't have a return (or ground) path so it has no effect on the capacitance, so it's just 2 capacitors (a and b) in series.