Among the following acids,the weakest one is

Mercury on heating with aqua-regia gives

Assertion $A$: $SF_6$ is highly stable.
Reason $R$: $SF_6$ is a gas.
The correct option among the following is:

Which is the best reducing agent?

$MnO_{2} + HCl \xrightarrow{\Delta} A_{(g)}$
$A_{(g)} + F_{2} (\text{excess}) \rightarrow B_{(g)}$
$B_{(l)} + U_{(s)} \rightarrow C_{(g)} + D_{(g)}$
The gases $A$,$B$,$C$ and $D$ are respectively

Which of the following halogens is most easily reduced?