Initial value $z=1$.
Let's check the values of the binary array $c=[1,0,1,1]$.
When $c[0]=1$ it is $z \leftarrow z^2 = 1$ and $z \leftarrow z\times a = 1\times a =a$
When $c[1]=0$ it is $z \leftarrow z^2 = a^2$ and $z \leftarrow z\times a$ is not evaluated.
When $c[2]=1$ it is $z \leftarrow z^2 = (a^2)^2=a^4$ and $z \leftarrow z\times a = a^4\times a =a^5$
When $c[3]=1$ it is $z \leftarrow z^2 = (a^5)^2=a^{10}$ and $z \leftarrow z\times a = a^{10}\times a =a^{11}$
Note that $(1011)_2=11$ from the binary array $c=[1,0,1,1]$.
So the question reduces to $a^{\text{DecimalValue(Binary Array }c)}\mathrm{~mod~}n$
$\therefore a^{11} \mathrm{~mod~}n = 2^{11}\mathrm{~mod~}8=0$
So the output is $0$.