The answer explanation isn't that great. Whether a ketone or alcohol has a larger dipole moment depends on the situation and ketones are quite electrophilic - hence the vast array of carbonyl chemistries that are found in nature.
A better explanation is this. Think about how TLC works. It's basically a column but on a thin piece of cardboard. The stationary phase is silica and the mobile phase is your solvent. If something sticks well to the silica, it'll have a smaller Rf because it won't want to move with the mobile phase. So ask yourself: what sticks best to silica?
If you can answer that question correctly, then it becomes obvious why the answer is so bad. It's not merely because the compound is polar that it sticks to silica - it's that it hydrogen bonds. Anything that can hydrogen bond will love silica. So the alcohol will stick more tightly than the ketone. This will narrow your choices down to C and D. From there, you'll have to pick the "best" one (not necessarily the correct answer choice in absolute terms but the one that is closest), which is D.