(A) Both Assertion (A) and Reason (R) are true and Reason (R) is the correct explanation of Assertion (A).
The carbon-carbon bond is very strong and hence stable, which directly gives rise to large numbers of compounds with many carbon atoms linked to each other (catenation).
Source: Chapter 4, Section 4.2 Versatile Nature of Carbon
The textbook explicitly states: "The carbon-carbon bond is very strong and hence stable. This gives us the large number of compounds with many carbon atoms linked to each other." This means R directly explains A, making option (A) correct. Students should note the key link: strong C–C bond → stable catenation → millions of carbon compounds.