Question regarding the Double The Raise rule...

[Areola]

Which is the correct re-raise regarding the "double the raise" rule in no-limit (for this purpose, no-limit hold'em) games.

Blinds $50/$100

Initial raise is $250 (minimum re-raise is $400).

If the next player re-raises to $500 from the initial raise of $250, what's the minimum raise by the next player?

a) $750 ($250 + $500)
There's a raise of $250, which will be added to the
current bet of $500, making the total $750. This is the
way I understand the "double the raise" rule.

b) $900 ($400 + $500)
There's a re-evaluation of the raise from the big blind
of the $100 is $500-$100=$400. On top of the $500
re-raise would mean the next minimum re-raise is $900