If the tension in the cable supporting an elevator is equal to the weight of the elevator, the elevator may be

For lift moving with upward acceleration, $T-mg=ma\Rightarrow T=m\left(g+a\right)>W$
  Option (a) is wrong.
 For lift moving with downward acceleration, $mg-T=ma\Rightarrow T=m\left(g-a\right)<W$,   Option (b) is wrong.
For lift moving up or down with no acceleration,

$T=mg=W$,     Option (c) is correct. Option (d) is correct.