Carbohydrates

Lactose is a disaccharide composed of two monosaccharides: glucose and galactose. Specifically, it is formed by the condensation of one molecule of -Galactose and -Glucose. In lactose, the glucose unit is in the -configuration, while the galactose is in the -configuration, which is characteristic of lactose.
Thus, the correct combination is -Galactose and -Glucose.

In the reaction, the rate-determining step involves the formation of a carbocation intermediate. The reactivity of the compound is largely dependent on the stability of the carbocation formed. The more stable the carbocation, the more reactive the compound will be in an reaction.
The general order of reactivity for reactions is: Where represents a tertiary carbon, represents a secondary carbon, and represents a primary carbon. - Compound A ( ): This is a primary alkyl halide, which forms a less stable primary carbocation, making it less reactive in . - Compound B ( ): This is a primary alkyl halide, similar to compound A. - Compound C ( ): Again a primary halide, yet it would still undergo in the presence of stabilizing effects from a group like , which increases its reactivity. - Compound D ( ): This is a tertiary halide, and tertiary carbocations are more stable, so compound D is most reactive in reactions. However, compound C is more reactive. Thus, the compound in option is most reactive in an reaction.