Step 1: Recall the relation between time and fraction of oscillation.
In simple harmonic motion, the time taken to complete a given fraction of one oscillation is directly proportional to the time period.
Thus, for completing
\[
\frac18
\]
of an oscillation,
\[
t=\frac{T}{8}.
\]
The amplitude does not affect the time taken.
Step 2: Calculate the required times.
For particle \(A\),
\[
t_A=\frac{8}{8}=1\,\text{s}.
\]
For particle \(B\),
\[
t_B=\frac{12}{8}=\frac32\,\text{s}.
\]
Step 3: Find the ratio.
Therefore,
\[
t_A:t_B
=
1:\frac32
=
2:3.
\]
Hence,
\[
\boxed{t_A:t_B=2:3.}
\]
Therefore, the correct option is \(\boxed{(D)}\).