Answer
(a) periodic $p=2$
(b) not periodic
(c) not periodic
(d) periodic $p=3$
Work Step by Step
(a) This is a periodic function as it satisfies $f(t+p)=f(t)$ with $p=2$
(b) This is not a periodic function as it does not satisfies $f(t+p)=f(t)$, the amplitudes decay with increasing x.
(c) This is not a periodic function as it does not satisfies $f(t+p)=f(t)$, the function jumped across the y-axis.
(d) This is a periodic function as it satisfies $f(t+p)=f(t)$ with $p=3$