Vanessa,
As far as I know, there is not necessarily any up-front legal requirement for a buyer to honor stock options of the acquired firm.

HOWEVER, from a practical perspective, they are virtually ALWAYS honored. This is true both because the original firm's stock option agreement will typically have acceleration provisions that provide the value of the options (and often fully vest any not-yet-vested options) in the event of a takeover, and also because the buying firm nearly always agrees to fully honor the commitments of the preceding firm. The reason is relatively straightforward - if the employee fears a takeover and that the options will not retain their value through a takeover, the employee is demotivated, and may leave for a competitor. So it's in the original firm's interests to secure the compensation of their key employees to retain them. And it's in the acquiring firm's interests (at least in most cases) to likewise secure the compensation of key employees, otherwise you end out buying a firm and getting none of the talent that got the firm where it is!

I hope that helps a little!