The answer is (d); the surface will appear to have imploded inwards, in the shape of a circle above the slit.
When air is released from under the sand surface, it blows the sand away until the sand particles no longer roll back down the sloped sides, creating an inverted cone shape. This is similar to what happened, all of a sudden, when the ball bearing "meteor" hit the sand surface (see Answer #353). Click your mouse on the photograph above to see the process.
This is another way in which craters can develop on planets or moons, e.g. volcanic craters. The characteristic shape of the crater gives scientists a hint as to the process by which it originated.
