Concept:
For cohesive soils (clay), the settlement of a footing is directly proportional to its width.
The relationship between footing settlement and plate settlement is
\[
\boxed{
S_f=S_p\left(\frac{B_f}{B_p}\right)
}
\]
where
\[
S_f=\text{Settlement of footing},
\]
\[
S_p=\text{Settlement of plate},
\]
\[
B_f=\text{Width of footing},
\]
\[
B_p=\text{Width (diameter) of plate}.
\]
Step 1: Write the given data.
\[
S_p=18\text{ mm}
\]
\[
B_p=300\text{ mm}=0.3\text{ m}
\]
\[
B_f=2\text{ m}
\]
Step 2: Apply the settlement relationship.
\[
S_f
=
18\left(\frac{2}{0.3}\right)
=
18\times6.667
=
120\text{ mm}
\]
Hence,
\[
\boxed{S_f=120\text{ mm}}
\]
Therefore, the correct option is
\[
\boxed{(D)\;}
\]