The presence of \(SO_2\) is not detected by

Turning of acidified potassium dichromate solution green

The correct answer is option 3. Turning of acidified potassium dichromate solution green.

When sulfur dioxide (\(SO_2\)) is passed through acidified potassium dichromate solution (\(K_2Cr_2O_7\)), it reduces the dichromate ions (\(Cr_2O_7^{2-}\)) to chromium ions (\(Cr^{3+}\)). This reduction is indicated by a color change from orange to green.

The overall reaction is represented as:

\[ 6H^+ + 6e^- + Cr_2O_7^{2-} \rightarrow 2Cr^{3+} + 7H_2O \]

The green color indicates the reduction of chromium(VI) to chromium(III), confirming the presence of \(SO_2\). The green coloration is a result of the formation of chromium(III) ions in the acidic medium.
The correct answer is indeed (3) Turning of acidified potassium dichromate solution green.