Which one of the following alkenes will react fastest with $H_2$ under catalytic hydrogenation condition?

Arrange the following compounds in decreasing order of their heats of combustion:

The octane number of diesel is increased by adding ..........

The number of sigma $(σ)$ and pi $(π)$ bonds in pent$-2-$en$-4-$yne is

An organic compound $C_4H_6$ on ozonolysis gives $HCHO$,$CO_2$,and $CH_3CHO$. The compound will be:

Which is the most precise designation of stereochemistry for the products formed in the electrophilic addition of $DBr$ to $1$-methylcyclohexene? ($D = {}^2H$,an isotope of hydrogen)