How many isomers are possible for pentyne $(C_5H_8)$?

$CH_3-C \equiv C-CH_3$ $\xrightarrow{H^{+}/KMnO_4, \Delta} A$ $\xrightarrow{(1) LiAlH_4, (2) H^{+}} B$
$B$ will be

Which of the following bonds makes a hydrocarbon most reactive?

Hex$-1-$yne on reaction with $Br_2$ (excess)/ $CCl_4$,gives

Which combination is best for the preparation of the compound $(A)$ shown below?

When acetylene is passed into dilute sulphuric acid containing $Hg^{2+}$ ions,the product formed is