Correct Answer - Option 2 : the same whether computed from primary or secondary side
Per unit system:
It is usual to express voltage, current, voltamperes and impedance of an electrical circuit in per unit (or percentage) of base or reference values of these quantities.
The Per Unit value of any quantity is defined as
PU value = actual value/base value
Per unit system in transformers:
The per unit impedance of a transformer is the same whether computed from primary or secondary side so long as the voltage bases on the two sides are in the ratio of transformation (equivalent per phase ratio of a three-phase transformer which is the same as the ratio of line-to-line voltage rating).
Transformation ratio of transformer is given by K = V2/V1 = E2/E1 = N2/N1.
Where N1 is the number of primary turns
V1 is the primary voltage
N2 is the number of secondary turns
V2 is the secondary voltage