None of the following has an sp3 carbon bonded to four different groups. Which of them are chiral (optically active as single enantiomers)?

A chiral axis makes penta-2,3-diene chiral: each terminal carbon has two different groups (H and $\mathrm{CH}{}_3$) and the two end planes are perpendicular. 3-Methylbuta-1,2-diene has two identical groups on each end carbon of one terminus, so it has a mirror plane. The hindered tetra-ortho-substituted biphenyl (with each ring having two different ortho groups) is chiral, whereas 4,4'-dibromobiphenyl is achiral.