**Category:** Electromagnetism  
**Subtype:** Alternating Current (AC) Circuits  
**What the problem is about:** Find the inductive reactance (cảm kháng) of a pure inductor in an AC circuit.

**Summary:** The problem involves an AC voltage applied to a pure inductor, and you need to calculate the inductive reactance based on the given parameters. The voltage is described by the equation \(u=U_{0} \cos (100 \pi \mathrm{t})\) and the inductance \(L\) is given as \(\frac{0,2}{\pi} \mathrm{H}\). The options provided are different values of reactance in ohms.