I think you would call them P*-Headers, where the * designates which one exactly (5/2/3/6).
What you call Opportunity B is known as the P5-Header, and you can use it just like the GPIO-Pins next to it (see also this link).
Opportunity A (P6) can be used to reset your Raspberry Pi: Shorten it it reset. Be careful, though, as doing this while your SD-Card is active (writing), may damage the card.
Opportunity C consists of two seperate Headers: P2 (closer to the GPIO) and P3 (closer to the USB/network connectors).
P2 is used during production of the board and is called "VideoCore JTAG", but cannot be used by the arm processor.
P3 is called "LAN9512 JTAG" and is used by manufacturers to debug a device.
You can find some more information about so called "low-level-Peripherals" here.