Answer : (A) Both A and R are true and R is the correct explanation of A
Step 1 — Test the Assertion
D-(+)-Glucose is indeed dextrorotatory :
$$[\alpha]_D = +52 \cdot 5^\circ$$
It rotates plane-polarised light to the right (clockwise). The assertion is true.
Step 2 — Test the Reason
Both parts of the reason are correct :
'(+)' means dextrorotatory — it is an experimental observation, measured with a polarimeter. A '(−)' prefix would mean laevorotatory.
'D' represents the configuration — it refers to the spatial arrangement of groups at the highest-numbered chiral carbon (C-5 in glucose), compared with the reference compound glyceraldehyde. If the $-OH$ there lies on the right in the Fischer projection, the sugar is D.
The reason is true.
Step 3 — Check whether R explains A
The reason defines the very notation used in the assertion. Once we know that '(+)' means dextrorotatory, the statement "D-(+)-glucose is dextrorotatory" follows immediately from its name.
R therefore explains A directly.
Step 4 — Note the crucial point about independence
The two prefixes are completely independent of one another. A D sugar may be either dextro- or laevorotatory.
The clearest illustration : D-(−)-fructose. It has the D configuration at C-5, yet rotates light to the left $([\alpha]_D = -92 \cdot 4^\circ)$.
This is exactly why both prefixes must be stated — one gives the configuration, the other the observed rotation, and neither can be deduced from the other.
