[Solution from source] The dee voltage frequency must equal the cyclotron frequency ν = eB/(2πm), so B = 2πmν/e. At the dee radius R, v = eBR/m = 2πνR, so K = ½mv² = ½m(2πνR)² = 2π²mν²R². B = mν/e misses the 2π factor between ν and angular frequency. Answer (4).