Suppose the FED engaged in open market operations and sold a portion of its government securities. What effect would this have on federal funds rate?

The federal funds rate and the open market operations do not have a direct relation with one another. But, the open market operations affect the money supply and then the money supply affects the federal funds rate.

If the Fed engages in the open market operations to sell the government securities, then it implies, that is would get cash money in return, which implies that, the supply of money in the economy would be reduced.

The federal funds rate is fixed by the central bank. If the supply of money is reduced, then to bring the money market into equilibrium, the central bank needs to increase the federal funds rate, so that the demand for money do not increase.