When a spring is cut into 'n' equal parts, the spring constant of each part becomes 'nK'. Here n=4, so K' = 4K. The new time period T' = 2π√(m/K') = 2π√(m/4K) = (1/2) * 2π√(m/K) = T/2. So, T' = 5/2 = 2.5 S.