Step 1: Understanding the Concept:
The question asks to identify the philosophical system where compassion and friendliness are the primary ethical virtues.
Step 2: Detailed Explanation:
Let's examine the core virtues of each philosophy:
1. Buddhism: Two of the most central concepts in Buddhist ethics are \textit{Karuṇā} and \textit{Mettā}. \textit{Karuṇā} is deep compassion for all suffering beings, and \textit{Mettā} is loving-kindness or friendliness towards all beings. These are two of the four "Brahmavihāras" (sublime states) and are fundamental to the Buddhist path. This is a perfect match.
2. Jainism: The paramount virtue in Jainism is \textit{Ahiṃsā} (non-violence) in thought, word, and deed. While compassion is a part of this, the core emphasis is on non-harm.
3. Sikhism: Key virtues include selfless service (\textit{Sevā}), truthfulness (\textit{Sat}), contentment (\textit{Santokh}), and compassion (\textit{Dayā}). While compassion is important, it is part of a larger set of core principles.
4. Christianism (Christianity): The three theological virtues are Faith, Hope, and Love (\textit{Agape}). Love is central, but the pairing of "compassion and friendliness" is most characteristic of Buddhist terminology.
Step 3: Final Answer:
The philosophy that most explicitly and centrally propounds compassion (Karuṇā) and friendliness (Mettā) as its most important ethical virtues is Buddhism.