a) $600 billion
  b) $300 billion
  c) $0

And that's assuming the company is being liquidated. Most companies with viable businesses are worth a lot more than their assets because the big part of the valuation is assumed future earnings.

But if it was cash and debts instead of assets and debts, the company wouldn't be worth $300 billion to bondholders?

If you give a teller $1 for 4 quarters, you don't say that you spent $1.

I currently am worth $0 to that bondholder.

I spend one of those borrowed dollars on a chocolate bar.

Now I'm worth $1 to that bondholder??

The spending is done with a completely different person than the bondholder.

I'd love to create lots of value for you by borrowing money from you. I would even consent to not destroy any of that value through making payments.

The bond is worth $300b to the bondholders, but only because it is expected to be paid back and is secured by the $300b assets. You can prove that by assuming those assets disappeared: how much is the bond (or the company) worth to the bondholders then?

The intrinsic value of a company's equity often differs from the book value of equity.

The book value debt can also differ from the market value.

Learning some financial accounting concepts should help you understand this better.