A transformer is a device which converts:
A. Low voltage at low current into high voltage at high current
B. High voltage at low current into low voltage at high current
C. High voltage at high current into high voltage at high current
D. Electrical power into mechanical power

Hint: Transformer has two coils that are secondary and primary coil. From Faraday's law of induction, use the relation between voltage and current of secondary coil as well as primary coil.

Complete answer:
A transformer is an electrical device which transfers electrical energy from one circuit to another. It is a device that can increase or decrease the voltage of the circuit as per requirement. The transformer used to get the voltage of the secondary coil greater than the voltage of the primary coil is called the Step-up transformer. The transformer used to get the voltage of the secondary coil lower than the voltage of the primary coil is called the Step-up transformer. It works on the principle of Faraday's induction.
Derived equation from Faraday's law of infection is given by,
${V}_{s}.{I}_{s}= {V}_{p}.{I}_{p}$
From the above relation it can be understood that voltage is inversely proportional to current but voltage of secondary coil is proportional to voltage of primary coil. So, when the voltage of the secondary coil is high, current is low and simultaneously the voltage of the primary cool is low and current is high.
Thus, we can say a transformer is a device which converts high voltage at low current into low voltage at high current.
Hence, the correct answer is High voltage at low current into low voltage at high current.

So, the correct answer is “Option B”.

Note:
The ratio of voltages across both the coils i.e. primary and secondary coil depend on the number of turns in each coil. In expression form it is given as,
$\dfrac {{N}_{s}}{{N}_{p}}= \dfrac {{V}_{s}}{{V}_{p}}$
Where, ${N}_{s}$ and ${N}_{p}$ are the number of turns in secondary and primary coil respectively.
           ${V}_{s}$ and ${V}_{p}$ are the voltages in secondary and primary coil respectively.