Time period of a pendulum is given by --
![T=\text{ 2}\pi\sqrt[\placeholder{⬚}]{(l)/(g+a)};](https://img.qammunity.org/2023/formulas/physics/college/bqc8jgq35glpvb7fxf6ecz45dby1puooqv.png)
According to problem
When it is at rest, it oscillates with a period of 1 second. When it is accelerating in an elevator it oscillates with a period of 2 seconds.
From above equation we can conclude that time period of pendulum depends on acceleration and it is inversely proportional .
So its is a Pendulum.
final answer is :- pendulum