It is designed to be a not well posed question. The "correct answer" to this simple question should be to reference:
Bertrand's Paradox and Maximum Ignorance Principle.
They're looking for you to think outside of the box and apply theory.
How hard is the coin? How elastic? How hard/elastic is the table? How sharp are the edges of the coin. Is the weight of the coin evenly distributed? Is the coin coated? Will the coin be mechanically flipped? How high? How much spin? Spin on which axis? Will be be randomly flipped?
In practice you'll find the solution lies somewhere between a thickness of .35 and .6 of the diameter.