Solution

Part ,a) Why is it dangerous to close a loop in the secondary?

Because a single turn has very low resistance and, although the voltage induced in it is small, the current surges (I = V/R). This large current overheats it and can melt the insulation or the copper, damaging the transformer.

Part ,b) Why does the primary get damaged?

This happens because The magnetic flux in the core is set by the primary source. To compensate for the huge current in the secondary and maintain that flux, the primary suddenly draws an equally large current, which can burn out its own winding. It is a "reflection" of the fault on the other side.