What is the sampling rate in this case? A signal $x=\sin(\pi t/4)$
sampled at every $t=1$ sec.
so $T=1$ sec
Sampled signal is then $x=\sin(\pi n/4)$, 
What is the sampling rate in this case?, 
And according to Nyquist sampling rate what should be the minimum sampling rate to sample this signal?
For the sine wave $\sin ((\pi/4)t)$ , the minimum sampling frequency is greater than 1/4 Hz. That amounts to a sampling period less than 4 seconds. In your case, sampling period of 1 second is therefore a valid choice.