(a)
Glucose gives silver mirror with ammoniacal silver silver nitrate because of presence of $–CHO$ group (aldehyde group) in the structure of glucose.
$CH_{2}OH\ (CHOH)_{4}CHO + Ag_{2}O\ \rightarrow CH_{2}OH\ (CHOH)_{4}COOH\ + \ \ \ \ \ 2Ag\ \downarrow$
Gluconic acid silver mirror