Electro-magnetic relays may be operated by:

Electro-magnetic relays can indeed be operated by various principles, which makes the choice of "Any of the above" the correct answer.

Electro-magnetic attraction refers to the mechanism where an electromagnet pulls a movable armature toward it when the coil is energized. This is the most common operation principle for electro-magnetic relays, where the attraction creates mechanical movement to open or close contacts.

Electro-magnetic induction can also be utilized, particularly in certain types of relays where a changing magnetic field induces a current in a coil, which then controls the relay operation. This method might not be as common in traditional relays but can be applied in specific relay designs that incorporate inductive principles.

The thermal effect is another operating principle found in some relays, usually referred to as thermal relays. They operate based on heat generated by the current passing through a conductor, causing it to expand and move a mechanism to open or close the contacts.

Each of these operating principles demonstrates the versatility of relay design, allowing them to function based on electro-magnetic attraction, induction, or thermal effects. Therefore, the answer that encompasses all these methods is the most comprehensive and accurate.