b
In case of \(\mathrm{sp}^{3}\) hybridised carbon bond angle is \(109^{\circ} 28^{\prime} ; \mathrm{sp}^{2} ; \mathrm{sp}^{2} ;\) hybridised carbon, bond angle is \(120^{\circ}\) and \(sp\) hybridised carbon, bond angle is \(180^{\circ}\)
So, only
\(\mathrm{CH}_{3}-\underset{\mathrm{sp}}{\mathrm{C}} \equiv \underset{\mathrm{sp}}{\mathrm{C}}-\mathrm{CH}_{3}\) is linear.