A loosely wound helix made of stiff metal wire is mounted vertically with the lower end just touching the mercury in a dish. When the current from a battery is started in the metal wire through  the mercury, then the spring

Given Data:

A loosely wound helix made of stiff metal wire is mounted vertically with the lower end just touching the mercury in a dish. 

Detailed Solution:

When lower end comes into contact then spiral wire rings get attracted to each other

end get compressed and looses contact. As they become normal again current flow take place  making it to compress again. In this way it executes oscillatory motion

 Hence, the correct option is A.