= Solution
For <hexa-1,5-diyne> in the extended anti conformation drawn, inversion at the midpoint of the central <carbon>–<carbon> bond exchanges the two identical ends. A twofold axis through that midpoint perpendicular to the <carbon> skeleton plane, together with the mirror plane of that skeleton, completes the <C2h molecular point group>. The methylene hydrogens occur in the corresponding mirror-related pairs. Thus the answer is \b[$C_{2h}$]. The whole molecule is not linear simply because its terminal triple-bond fragments are linear; the conformation is essential to this assignment.
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