This question assesses the understanding of the word 'voluntary' and its correct usage. The word 'voluntary' means done willingly or of one's own free will. Option B is correct because it uses 'voluntary' in a negative context to mean that the contribution was not made by choice. Option A is incorrect because it contradicts the meaning of 'voluntary'. Option C is grammatically incorrect and does not use 'voluntary' properly. Option D should be 'voluntarily' instead of 'voluntary' to correctly modify the verb 'resigned'.