Which of the following has the lowest $pK_a$ value?

The number of stereoisomers possible for $1,2-$dimethylcyclopropane is:

The number of stereoisomers obtained by bromination of $trans-2-butene$ is

The unknown compound $(A)$ is:

Match the terms mentioned in Column-$I$ with the terms in Column-$II$.

Column-$I$	Column-$II$
$(A)$ Carbocation	$(1)$ Cyclohexane and $1$-hexene
$(B)$ Nucleophile	$(2)$ Conjugation of electrons of $C-H$ $\sigma$-bond with empty $p$-orbital present at adjacent positively charged carbon
$(C)$ Hyperconjugation	$(3)$ $sp^{2}$ hybridised carbon with empty $p$-orbital
$(D)$ Isomers	$(4)$ Ethyne
$(E)$ $sp$ hybridisation	$(5)$ Species that can receive a pair of electrons
$(F)$ Electrophile	$(6)$ Species that can supply a pair of electrons

$A$ compound was synthesized by a student,but its structure was not identified. However,his instructor told him that it was a meso compound with $5$ carbons and $2$ stereogenic centers. Which of the following structures should the student consider as possibilities for his compound?