**The correct choice is A.**

It is given that normally the block slides down the incline with the constant velocity, so the acceleration due to the friction and the component of gravity parallel to the incline is 0:

`F_f - mgsintheta=0` . ( `F_f` is the force of friction.)

Now the force F is applied horizontally, pressing the block against the incline, so it's component parallel to the incline is in the same direction as the friction: `Fcostheta` .

Now the second Newton's Law looks as follows:

`Fcostheta+F_f - mgsintheta = ma` . Here a is directed up the incline, so the block is decelerating.

Since `F_f - mgsintheta = 0` as stated above,

`Fcostheta = ma`

`a = (Fcostheta)/m` .

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