The correct statement regarding the following compounds is:

The total number of stereoisomers that can exist for $M$ is:
$M$ is a bicyclic ketone with a methyl group at the bridgehead and two methyl groups at the bridge carbon.

The well-known compounds, $(+)$-lactic acid and $(-)$-lactic acid, have the same molecular formula, $C_{3}H_{6}O_{3}$. The correct relationship between them is

Explain the following terms:
$(i)$ Plane polarised light
$(ii)$ Optical activity
$(iii)$ Chirality
$(iv)$ Chiral centre (atom)
$(v)$ Enantiomers
$(vi)$ Diastereomers
$(vii)$ Meso compounds
$(viii)$ Plane of symmetry and Achiral molecule
$(ix)$ Centre of symmetry

Which among the following compounds is $\underline{NOT}$ optically active?

Which dimethylcyclobutane is optically active?