Not able to upload the image but the question was to state the strongest type of intermolecular force in HI (hydrogen iodide). The marking scheme says vdW forces - why is it not permanent dipole-dipole?
In general, although dipole-dipole is usually more significant than the vdW, for some molecules (& because I is quite large and so has a large electron cloud), the vdW will actually be stronger than the permanent dipole-dipole. That is what this sheet says https://web.chem.ucsb.edu/~genchem/Chem1CS03/moreinfo/trends_in_boiling_points_.pdf
To be honest, I think I would have chosen the wrong answer as well if I saw that question - it might be worth just going over it with you teacher.
In general, although dipole-dipole is usually more significant than the vdW, for some molecules (& because I is quite large and so has a large electron cloud), the vdW will actually be stronger than the permanent dipole-dipole. That is what this sheet says https://web.chem.ucsb.edu/~genchem/Chem1CS03/moreinfo/trends_in_boiling_points_.pdf
To be honest, I think I would have chosen the wrong answer as well if I saw that question - it might be worth just going over it with you teacher.