Observe that
\[
A^{2026}-A^{2025}
=
A^{2025}(A-I).
\]
Taking determinant,
\[
\left|A^{2026}-A^{2025}\right|
=
|A^{2025}|\;|A-I|.
\]
Now,
\[
A-I=
\begin{bmatrix}
2025&2025&2024\\
2025&2023&2023\\
2024&2023&2021
\end{bmatrix}.
\]
Since
\[
R_1-2R_2+R_3=0,
\]
the rows are linearly dependent.
Hence,
\[
|A-I|=0.
\]
Therefore,
\[
\boxed{\left|A^{2026}-A^{2025}\right|=0.}
\]
Thus,
\[
\boxed{(D)}
\]