The most suitable reagent for the following conversion is-

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Accepted Answer

1.  **Identify the Reagent:** The reagent specified in Option 3 is dihydrogen gas ($H_2$) in the presence of palladium supported on charcoal ($Pd/C$) and deactivated with quinoline or sulphur. This specific combination is known as **Lindlar’s catalyst** [NCERT 11th, Ch 13, Sec 13.3.4; Source 125].
2.  **Function:** Lindlar's catalyst is used for the **partial reduction** of alkynes to **cis-alkenes**. The reaction involves the syn-addition of hydrogen.
    $$ RC\equiv CR' + H_2 \xrightarrow{Pd/C, 	ext{quinoline}} 	ext{cis-alkene} $$
3.  **Context (NEET 2019):** In the original exam question, the reaction scheme (missing from the input text) depicted the conversion of **but-2-yne** to **cis-but-2-ene**.
4.  **Analysis of Other Options:**
    *   $Na/	ext{liquid } NH_3$ (Birch reduction) reduces alkynes to **trans-alkenes** .
    *   $Hg^{2+}/H^+, H_2O$ is used for the hydration of alkynes to form ketones or aldehydes.
    *   $Zn/HCl$ is a reducing agent typically used for alkyl halides or Clemmensen reduction.

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