Multiple Choice:

a)Nothing

As the ships increase speed, they move like one long ship. In the frame of either ship, the ships are at rest and the Universe is accelerating past them. In the station frame, the "one long ship" length contracts; that is, the string length contracts but so does the distance between Ships A and B in exactly the same way. The string tension is unchanged.

b)String breaks

Because the ships start out together from rest with the same acceleration, they will always have the same velocity and hence the same separation as seen from the station. At high speeds, the string will length contract, but the distance between the Ships will not. The string breaks. As seen from either Ship, the distance between the ships is increasing. Again, the string breaks.

c)String goes slack

The space between the ships length contracts (so does each individual ship) as seen from the station because the ships are moving relative to the station. The string does not length contract because the string is at rest relative to either ship.