Answer: C. Both electrically and magnetically
In an auto transformer the primary and secondary are both electrically and magnetically coupled.
An auto transformer is a one winding (or) single circuit transformer, in which a portion of the winding is common for both high voltage and low voltage winding. And this entire winding will be placed on a single magnetic core as shown in the figure below.
Unlike a two-winding transformer in an auto transformer there is no electrical isolation between two windings, so, in an auto transformer the primary and secondary windings are coupled both magnetically and electrically.
In a two-winding transformer power is transferred between two windings through a magnetic link by a process called as induction. But in an auto transformer power transfer between primary and secondary windings will be both electrically through the conduction process and magnetically through the induction process.
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