Which of the following is also called as ‘series transformer’? 

The correct answer is option 2):(Current transformer )

Concept:

• A current transformer's primary coil is always connected in series with the main conductor giving rise to it also being referred to as a series transformer. The nominal secondary current is rated at 1A or 5A for ease of measurement
• A current transformer is also called  ‘series transformer’
• A current transformer (CT) is an instrument transformer in which the secondary current is substantially proportional to the primary current and differs in phase from it by ideally zero degrees.
• The primary current of the CT does not depend upon whether the load or burden is connected to the secondary & also not on the impedance value of the burden.