Meso-tartaric acid is optically inactive due to the presence of

Which of the following compounds is not chiral?

Explain or define the following terms:
$(i)$ Optical activity
$(ii)$ Polarimeter
$(iii)$ Dextrorotatory isomer

Which of the following compounds are optically active?
$(I)$ $1,1-$dimethyl$-4-$chlorocyclohexane
$(II)$ $4-$chlorocyclohex$-1-$ene
$(III)$ penta$-2,3-$diene

How many stereoisomers are possible for $2$-hydroxycyclopentanethiol?

Which of the following is optically active?