Which of these materials requires the least value of magnetic field...

The least value of magnetic field strength required to magnetize is nickel because it is a ferromagnetic material. $Ni$ is a ferromagnetic substance which gets strongly magnetized when placed in an external magnetic field. When external magnetic field ${B}_{0}$, is applied the magnetic domains orient themselves in the direction of magnetic filed and grow in size.

Hence, $Ni$ requires least value of magnetic strength. $NaCl$ is a diamagnetic material. Its field lines are repelled.