Which of the following is a carcinogen?

In which of the following compounds does the $C-H$ bond exhibit the lowest bond dissociation energy?

The enthalpy of hydrogenation of benzene is $\Delta H_1$ and for cyclohexene is $\Delta H_2$. Then the resonance energy of benzene is:

Reaction of $SO_3$ is easier in:

In the mixture of conc. $H_2SO_4$ and $HNO_3$,the nitrating species is:

Which of the following molecules,in pure form,is (are) unstable at room temperature?
$(A)$ $1,3-$cyclohexadiene
$(B)$ Cyclobutadiene
$(C)$ Cyclopentadienone
$(D)$ Cycloheptatrienone (Tropone)