Which one of the following statement is true for transition eleme…

$\begin{align}
\begin{matrix}
O\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\, \\
||\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\, \\
H-O-C-C{{H}_{2}}-C{{H}_{2}}-CH-C{{O}_{2}}H \\
\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,| \\
\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\underset{\oplus }{\mathop{N}}\,{{H}_{3}} \\
\end{matrix}\,\,\,\,\,(p{{K}_{a}}=2) \\
(p{{K}_{a}}=4)\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,(p{{K}_{a}}=9) \\
\end{align}$

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Given below are two statements:

Statement $(I)$: A solution of $\left[\mathrm{Ni}\left(\mathrm{H}_2 \mathrm{O}\right)_6\right]^{2+}$ is green in colour.

Statement $(II)$: A solution of $\left[\mathrm{Ni}(\mathrm{CN})_4\right]^{2-}$ is colourless.

In the light of the above statements, choose the most appropriate answer from the options given below:

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Which of the following reacting is used for production of $K_2Cr_2O_7$ ?

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The most stable ion is

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Consider the reaction of $HI$ with the following :

Which forms di-iodide on reaction with $HI $ (excess) ?