The maximum number of compounds that have the same net dipole moment is? The dipole moment μ of C−Cl = 1.5D
Case (3) has net zero moment due to the symmetrical molecular structure of 1,4 - Dichlorobenzene.
In case (5), the compound is 1,2,3,5 - Tetrachlorobenzene. If you assume that there is no Chlorine at the 2nd carbon atom, then we get a perfectly symmetrical 1,3,5 - Trichlorobenzene which will have 0 dipole moment. For 1,2,3,5 - Tetrachlorobenzene, the vector components of dipole moments from C−Cl bonds at 1,3,5 will cancel out. Effectively it will have the same net dipole moment as Ph−Cl. Again, if we discard the dipole moments due to the para-positioned Cl atoms of (5) (because they cancel each other out), then structure (5) becomes similar to (6).
In case (6), para-positioned Cl cancel out and net is the meta-isomer, just like case (5)
In case (4), para-positioned Cl cancel out and will be same as case (1)
Thus, (1) (4) (5) (6) all have the same similar dipole moment. You could verify by this by calculating them vectorially as well (for each case).