Went through an acquisition with Vmware... they converted the stock options to Vmware options that vested at the same rate and were worth the exact same based upon the acquisition price. If they had already vested, they just paid you out. This may depend on the deal though.
I've been through two acquisitions and part of the deal terms if the valuation of the acquiree's shares (which the cap table and liquidation preferences have a huge effect on).
First time my options were converted to options in the acquirer's company.
It depends on which type of options. The exercise date could be fixed or variable. But you could sell the options at its current market if there is a market.
Usually depends on the type of option. If they are 'double cliff' then the employee stock option will vest automatically, if they aren't then the emplyee will lose them.