Concept:
According to the Census of India 2011, the population density (persons/\(\text{km}^2\)) of these states is approximately:
\[
\begin{aligned}
\text{Sikkim} &\rightarrow 86,
\text{Mizoram} &\rightarrow 52,
\text{Manipur} &\rightarrow 122,
\text{Nagaland} &\rightarrow 119.
\end{aligned}
\]
Step 1: Arrange from lowest to highest density.
\[
\boxed{
\text{Mizoram}
<
\text{Sikkim}
<
\text{Nagaland}
<
\text{Manipur}
}
\]
Step 2: Convert into letter sequence.
\[
\boxed{
B \rightarrow D \rightarrow C \rightarrow A
}
\]
Hence,
\[
\boxed{\text{(A)}}
\]