Clarification

Option B is almost always impossible to apply to a whole building. If you have any interactive energy efficiency strategies like building envelope or lighting improvements you can't use Option B. Option C is typically by far the easiest one to apply assuming you have access to a reasonable data set of comparable building energy use. Whether you need to install submeters in the remaining spaces depends on the option you choose and the cost impacts. In a larger building Options B and D would likely requires some whole building submetering of building system like you have done in the common spaces. You always have the option of gathering the data through other means (by hand and with short term trending devices) but those come with a very high labor cost in many cases. The IPMVP tells you the required content of the M&V Plan for each Option.

Please not that Option C referenced in my comment above is no longer available for this credit.