You are absolutely correct. The answer is c) No answer until panel is metered?

Here's why: Diversity/Load Factor

Electrical loads rarely run at their peak simultaneously. Panel B might peak at 100A at 10 AM when office equipment is starting up. Panel C might peak at 150A at 4 PM when air conditioning is working hardest. Simply adding the peak values together overestimates the load on panel A.

Code Requirements

Electrical codes typically have demand factors or diversity factors that account for the non-coincidence of peak loads. These factors reduce the calculated load based on the type of loads involved.

Safety & Accuracy

Guessing at the load on panel A could lead to: Undersizing: If you underestimate, you risk overloading the panel, causing breakers to trip, and potentially creating a fire hazard. Oversizing: If you overestimate, you waste money on a larger panel, larger conductors, and larger overcurrent protection devices than you need.

Metering is the best way

You need to know the actual peak demand on panel A to properly size the panel and its feeders.Options a and b are incorrect because they do not account for the diversity/load factor.

In short, it's always best practice to meter the panel to obtain an accurate measurement of its peak demand. Flag for review