The linear molecule among the following is:

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1. **Hybridisation and Geometry:** The shape of an organic molecule is determined by the hybridisation of its carbon atoms [NCERT 11th, Ch 12, Sec 12.3].
   - **sp hybridisation:** Bond angle = 180° (Linear).
   - **sp² hybridisation:** Bond angle = 120° (Trigonal Planar).
   - **sp³ hybridisation:** Bond angle = 109.5° (Tetrahedral).
2. **Analyze Option A ($CH_3-C \equiv C-CH_3$, 2-Butyne):**
   - The central carbons ($C_2$ and $C_3$) are triply bonded. Triply bonded carbons are **sp hybridised**. This creates a linear arrangement of atoms along the $C_1-C_2-C_3-C_4$ axis ($180^\circ$ bond angle).
   - While the methyl hydrogens are tetrahedral, the carbon skeleton itself is linear [NCERT 11th, Ch 13, Sec 13.4.2].
3. **Analyze Other Options:**
   - **Option B ($CH_2=CH-CH_2-C \equiv CH$):** Contains $sp^2$ (trigonal planar) and $sp^3$ (tetrahedral) carbons, creating a bent/zigzag chain.
   - **Option C ($CH_3-CH_2-CH_2-CH_3$, n-Butane):** All carbons are $sp^3$ hybridised. The chain is zigzag (tetrahedral geometry at each carbon).
   - **Option D ($CH_3-CH=CH-CH_3$, 2-Butene):** Contains $sp^2$ carbons. The geometry around the double bond is trigonal planar, not linear.
4. **Conclusion:** 2-Butyne is the linear molecule.

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