What kind of forces arise from transient dipoles in atoms that induce transient dipoles in nearby atoms, produce an attractive force and are significant only over short distances (~500 pm)?

The correct answer is B, London dispersion forces. These are the weak attractions that appear when the electron cloud of an atom becomes momentarily lopsided, creating a temporary dipole which then induces a matching dipole in a neighbouring atom. Named after Fritz London, they are the weakest of the van der Waals forces and fall away very sharply with distance, so they count only over a few hundred picometres. They exist between all atoms and molecules and are the only forces holding non-polar substances such as helium, argon and methane together, which is why such gases liquefy only at very low temperatures. Their strength grows as the electron cloud gets larger. A is wrong because dipole-dipole forces act between molecules that already carry permanent dipoles. C is wrong because dipole-induced dipole forces need one permanent dipole to begin with. D is wrong because a hydrogen bond is a far stronger attraction, formed where hydrogen is bonded to nitrogen, oxygen or fluorine. Exam tip: strength order - hydrogen bond, then dipole-dipole, then London dispersion.