The greater the values of b and k, the greater the variation of b with z according to the magnetic induction intensity B=B0+kz, and the greater the variation of u with z according to U = BINQB, that is, the higher the sensitivity of the sensor (△U△z), so b is correct.
C. free electrons move in the hall element. According to the left-handed rule, electrons are biased towards the lower surface, so the upper surface potential is high. So, C is wrong.
So choose ab.