Which points on the potential energy diagram represent the eclipsed conformation of ethane and staggered conformation of propane respectively?

 

Amongst the given options, the compound(s) in which all the atoms are in one plane in all the possible conformations (if any), is (are):

Consider the following conformations of $3-$Aminopropanal.

Amongst the above conformations ($P, Q, R, S$) one of them is most stable.
This can be attributed due to-

$\text{Which of the following is/are the Newman projection formula of }2,3-\text{Dimethylbutane across }{\mathrm{C}}_2 \text{and} {\mathrm{C}}_3 \text{bond}?$

Correct relation between the above compounds:

Which of the following (are) correctly matched?