#### Answer

$-1,1,3,5,7,...$ Arithmetic.

#### Work Step by Step

After observing the sequence carefully, we notice that the difference between two consecutive terms is the same. $-1,1,3,5,7,...$
Here, ${{a}_{1}}=-1,{{a}_{2}}=1,{{a}_{3}}=3,{{a}_{4}}=5,{{a}_{5}}=7,...$
So,
$\begin{align}
& {{a}_{2}}-{{a}_{1}}=1-\left( -1 \right) \\
& =2 \\
& {{a}_{3}}-{{a}_{2}}=3-1 \\
& =2
\end{align}$
$\begin{align}
& {{a}_{4}}-{{a}_{3}}=5-3 \\
& =2 \\
& {{a}_{5}}-{{a}_{4}}=7-5 \\
& =2
\end{align}$
Therefore, the common difference is 2.
Hence, the sequence is Arithmetic.