To solve this problem, we need to analyze the given statements:$\newline$Assertion (A): All vertebrates are chordates but all chordates are not vertebrates.$\newline$Reason (R): Notochord is replaced by the vertebral column in adult vertebrates.$\newline$Let's break down each statement:$\newline$• Assertion (A):$\newline$ - Chordates are a phylum of animals that possess a notochord at some stage in their life cycle.$\newline$ - Vertebrates are a subphylum of chordates that have a vertebral column.$\newline$ - Therefore, all vertebrates are chordates, but not all chordates develop a vertebral column.$\newline$ - Examples of chordates that are not vertebrates include tunicates and lancelets.$\newline$ - Hence, Assertion (A) is correct.$\newline$• Reason (R):$\newline$ - In vertebrates, the notochord present during embryonic development is typically replaced by a vertebral column.$\newline$ - This replacement is a key characteristic that distinguishes vertebrates from other chordates.$\newline$ - Hence, Reason (R) is correct.$\newline$Now, let's determine if (R) is the correct explanation of (A):$\newline$• The reason explains why all chordates are not vertebrates.$\newline$• It highlights the transition from notochord to vertebral column, which is a defining feature of vertebrates.$\newline$Therefore, (R) is indeed the correct explanation of (A).$\newline$Thus, the most appropriate answer is Option 1: Both (A) and (R) are correct and (R) is the correct explanation of (A).