In the carbanion $\text{CH}_3-\text{C}\equiv\text{C}^-$, the terminal carbon atom is bonded to the adjacent carbon atom by a triple bond (which consists of one $\sigma$ bond and two $\pi$ bonds) and possesses one lone pair of electrons (indicated by the negative charge). The steric number for this carbon atom is the sum of the number of $\sigma$ bonds and the number of lone pairs, which is $1 + 1 = 2$. A steric number of 2 corresponds to $sp$ hybridisation. Therefore, the lone pair of electrons resides in an $sp$ hybrid orbital.