What is a meso compound and why it is optically inactive?  What is the condition for a compound be called meso compound?

The Fischer projection of $\mathrm{D}$-erythrose is shown below.

$\mathrm{D}$-Erythrose and its isomers are listed as $\mathrm{P},\mathrm{Q},\mathrm{R},$ and $\mathrm{S}$ in Column I. Choose the correct relationship of $\mathrm{P},\mathrm{Q},\mathrm{R},$ and $\mathrm{S}$ with $\mathrm{D}$-erythrose from Column II.

	Column - I		Column -II
P.		1.	Diastereomer
Q.		2.	Identical
R.		3.	Enantiomer
S.

Which of the following alkenes can generate optically active compounds upon hydrogenation?