Which one of the following is a periodic signal?

A. $${x_1}\left( t \right) = 2{e^{j\left( {t + \frac{\pi }{4}} \right)}}u\left( t \right)$$

B. $${x_2}\left( n \right) = u\left[ n \right] + u\left[ { - n} \right]$$

C. $${x_3}\left[ n \right] = \sum\limits_{k = - \infty }^\infty {\left\{ {\delta \left[ {n - 4k} \right] - \delta \left[ {n - 1 - 4k} \right]} \right\}} $$

D. $${x_4}\left( t \right) = {e^{\left( { - 1 + j} \right)t}}$$

Answer: Option C