Explanation

(a)
On hydrolysis with dilute aqueous sulphuric acid, sucrose gives a equimolar mixture of D-(+) glucose and D-(-)-fructose.
$C_{12}H_{22}O_{11} + \ H_{2}O\ \overset{\ \ \ \ \ H_{2}SO_{4}\ \ \ \ \ }{\rightarrow}\ {\ \ C}_{6}H_{12}O_{6}\ \ \ + \ \ \ \ C_{6}H_{12}O_{6}\$
Sucrose D-(+)glucose D-(+)fructose
1
Sucrose is dextrorotatory but after hydrolysis gives dextrorotatory glucose and laevorotatory fructose, laevorotatory fructose is more, so the mixture is laevorotatory.