The key here is that the particle is said to have constant kinetic energy. Since we must assume the mass is constant (since we are not talking relativity or nuclear reactions here), that means its speed is constant (since KE = ½mv2). If the speed is constant (which we call "uniform circular motion"), there is no force involved in speeding it up or slowing it down. Therefore, the only net force on it is the force keeping it traveling in a circle: the centripetal force.
So, if we solve for the centripetal force, we will have the net force. [note: this net force may be made out of one or more individual forces acting together, but we are not asked about that]
Getting the centripetal force with this information is tricky. But I will note that the expression for centripetal force (Fc) is:
Fc = (mv2)/r
where r is the radius of the circular path. You are given that radius. Then, notice that the "mv2" expression also appears in the definition of kinetic energy, which you are also given. You should be able to put all of that together to solve for the centripetal force, and thus the net force in this uniform circular motion.
I hope this helps! If you have any further questions or would like to check an answer, please let me know.