A stone of mass m is tied to a string and is moved in a vertical circle of radius r making n revolutions per minute.

Hi, I have a question and I hope anyone could answer it:

A stone of mass m is tied to a string and is moved in a vertical circle of radius r making n revolutions per minute. The total tension in the string when the stone is at its lowest point is 

(a) m{g + (π²n²r)/900}

(b) m(g + π nr²)

(c) m (g + π nr)

(d) m (g + n² + r²)