Concept:
The stability number is defined as
\[
\boxed{
N_s=\frac{c}{\gamma H}
}
\]
where
\[
c=\text{Cohesion},
\]
\[
\gamma=\text{Unit weight of soil},
\]
\[
H=\text{Height (or depth) of slope}.
\]
Step 1: Write the given data.
\[
c=1\text{ t/m}^2
\]
\[
\gamma=2\text{ t/m}^3
\]
\[
H=5\text{ m}
\]
Step 2: Calculate the stability number.
\[
N_s
=
\frac{1}{2\times5}
=
\frac{1}{10}
=
0.1
\]
Hence,
\[
\boxed{N_s=0.1}
\]
Therefore, the correct option is
\[
\boxed{(A)\;0.1.}
\]