Explanation
Best design practices say the VPN design should allow for summarization. With regards to D - sometimes you cannot avoid overlapping addresses as this is what is configured at client's end, and the only option is to hide the overlapping subnet behind NAT - based on experience (The author of this remark has 50x VPN tunnels and majority of them is using NAT, even if the subnet doesn't overlap, we want to hide our real IPs behind something else - extra security)