Which of the following has the greatest reducing power?

Among the following,the correct order of acidity is

In which of the following is silica $(SiO_2)$ soluble?

To purify fluorine gas,fumes of $HF$ are removed by

$A$ greenish yellow gas reacts with an alkali metal hydroxide to form a halate which can be preferentially used in fire works and safety matches,the gas and halate respectively are

Which one will liberate $Br_2$ from $KBr$?