During curdling of milk, what happens to sugar present in it?
Curdling of milk is caused due to formation of lactic acid by the bacteria present in milk. It is an example of denaturation of protein, i.e., when a protein is subjected is some physical or chemical chages. Hydrogen bond get disturbed. Globules unfold and helix uncoil and protein loss its biological acturty.
How do you explain the presence of five $-$OH groups in glucose molecule?
Glucose on reaction with acetic anhydride produces glucose pentaacetate.
This reaction explain presence of five $-$OH groups.
Why does compound (A) given below not form an oxime?
Compound (A) does not form an oxime on reaction with NH$_2$OH due to absence of CHO group 
group.
Why must vitamin $C$ be supplied regularly in diet?
Vitamin C is water soluble hence, they are regularly excreted in urine and can not be stored in our body, so, they are supplied regularly in diet.
Sucrose is dextrorotatory but the mixture obtained after hydrolysis is laevorotatory. Explain.
Sucrose is dextrorotatory. On hydrolysis, it produces a mixture of glucose and fructose having specific rotation $+52.5^{\circ}$ and $-92.4^{\circ}$. Thus, the respectively net resultant mixture become laevorotatory. Hence, the mixture is laevorotatory and product is known as invert sugar.