A transformer used to reduce the alternating voltage is:
A. Step-up transformer
B. Step-down transformer
C. Both step-up and step-down transformers
D. None of these.

Hint: At trivial times, before the invention of alternating current, only direct current was used. The major disadvantage with this current was that its percentage of loss was very high. Hence several power stations were constructed at little gaps of distances. Hence it was a very expensive process. After the invention of alternating current by Nicola Tesla, this scenario got changed completely. The advantage of this was that one can use step-up and step-down transformers to adjust the voltage as per the need.

Complete answer:

Step-up transformer – A transformer that is used to increase the voltage from primary to secondary winding is called step-up transformer. The key feature of this transformer is that it has more number of secondary winding turns than primary winding turns.
Step-down transformer – A transformer that is used to decrease the voltage from primary to secondary winding is called step-down transformer. The key feature of this transformer is that it has more number of primary winding turns than secondary winding turns.
Thus the definition itself suggests that Step down transformer is used to decrease the alternating voltage.

Hence option B. is correct.

Note:
The primary winding is the coil that draws power from the source. Hence the winding connected to the main source is the primary winding. Where-as secondary winding coil is that delivers the energy at the transformed or changed voltage to the load. So, the secondary winding coil is connected to the load. Note, the two windings are well separated to avoid the linking of internal flux.