What is the condition for a tree to be weight balanced. where a is factor and n is a node?

(a) weight[n.left] >= a*weight[n] and weight[n.right] >= a*weight[n].

(b) weight[n.left] >= a*weight[n.right] and weight[n.right] >= a*weight[n].

(c) weight[n.left] >= a*weight[n.left] and weight[n.right] >= a*weight[n].

(d) weight[n] is a non zero

My question is taken from Weight Balanced Tree topic in section Binary Trees of Data Structures & Algorithms I

This question was addressed to me during an interview.