To solve this problem, we need to analyze each option to determine which characteristic feature always holds true for the corresponding group of animals.$\newline \bullet$Option 1: Cartilaginous endoskeleton – Chondrichthyes$\newline$ - Chondrichthyes, also known as cartilaginous fishes, include sharks, rays, and skates.$\newline$ - A defining characteristic of this group is that they possess a cartilaginous endoskeleton.$\newline$ - This feature is always true for Chondrichthyes, making this option correct.$\newline \bullet$Option 2: Viviparous – Mammalia$\newline$ - While many mammals are viviparous (giving birth to live young), not all are.$\newline$ - Some mammals, like monotremes (e.g., platypus and echidna), lay eggs.$\newline$ - Therefore, this characteristic does not always hold true for all mammals.$\newline \bullet$Option 3: Possess a mouth with an upper and a lower jaw – Chordata$\newline$ - Chordata is a large phylum that includes animals with a notochord at some stage of development.$\newline$ - However, not all chordates possess a mouth with an upper and a lower jaw.$\newline$ - For example, jawless fish like lampreys and hagfish are chordates but lack jaws.$\newline$ - Thus, this characteristic is not universally true for all chordates.$\newline \bullet$Option 4: 3-chambered heart with one incompletely divided ventricle – Reptilia$\newline$ - Most reptiles have a 3-chambered heart, but crocodilians (a subgroup of reptiles) have a 4-chambered heart.$\newline$ - Therefore, this characteristic is not true for all reptiles.$\newline$Based on the analysis, the correct option is Option 1, as the characteristic of having a cartilaginous endoskeleton always holds true for Chondrichthyes.