Spring constant required for the stiff rod to remain stable when pivoted at one end

Question

1. A uniform stiff rod of length 300 mm and having a weight of 300 N is pivoted at one end and connected to a spring at the other end. For keeping the rod vertical in a stable the minimum value of spring constant K needed is                                                   

   

    

(a) 300 N/m                                                        (b) 400 N/m

(c) 500 N/m                                                        (d) 1000 N/m