Answer:
B, C, D, and E give positive tests.
Explanation:
Fehling's test is a test for reducing sugars, i.e., those that have a hemiacetal or potential aldehyde group (Fig. 1)
If the test is positive, a deep blue solution of complexed copper(II) ion is converted to a brick-fed precipitate of copper(I) oxide.
The equation for the reaction is
[tex]\rm\underbrace{\hbox{RCHO}}_{\hbox{aldehyde}} +\underbrace{\hbox{2Cu^{2+}}}_{\hbox{deep blue}} + 5OH^{-} \longrightarrow \, RCOO^{-} +\underbrace{\hbox{Cu_{2}\text{O}}}_{\hbox{brick-red ppt}} + 3H$_{2}$O[/tex]
Positive Tests
Lactose (Fig. 2) is a disaccharide of galactose and glucose. It is a reducing sugar because it has a hemiacetal unit, indicated by the red arrow.
Maltose (Fig.3) is a disaccharide of glucose. It is a reducing sugar because it has a hemiacetal unit, indicated by the red arrow.
Fructose (Fig. 4) is a ketose, but it tautomerizes in basic media to aldoses glucose and mannose. thus, it gives a positive Fehling's test.
Glucose (Fig. 1) gives a positive test, because it has a potential aldehyde group.
Negative tests
Sucrose (Fig. 5) is a disaccharide of glucose and fructose. It is not a reducing sugar, because it lacks a hemiacetal group. It has only acetal groups (blue arrows).
