Assertion (A): Dialects are the broad range of social as well as regional varieties.
This is true. Dialects indeed refer to variations in language that are influenced by both regional (geographical location) and social factors (class, group, or occupation). So, the assertion is correct.
Reason (R): A dialect describes variations not only at the phonological level, but also at the levels of lexis and syntax.
This is also true. Dialects typically show differences in:
Phonology: Sound or pronunciation differences.
Lexis: Vocabulary differences (e.g., "soda" vs. "pop" in different regions).
Syntax: Differences in sentence structure or grammar.
So, the reason is also correct.
Explanation of the Relationship Between (A) and (R):
While both (A) and (R) are independently true, the reason is not the explanation of the assertion.
Assertion (A) talks about the breadth of dialects (how they are both social and regional).
Reason (R) focuses on linguistic features of dialects (differences in phonology, lexis, and syntax).
Since the reason doesn't explicitly explain the connection between dialects being "social and regional varieties," it cannot be considered the direct explanation of the assertion.