Sentence Correction

A suspect will find it difficult to prove that a police officer used excessive force if there is a lack of some other witness to attest.

(a)
(b) unless there will be another witness to attest
(c) without another witness's attestation
(d) should there be no attestation from some other witness
(e) lacking another witness to attest

Answer : c

Can someone put their thoughts on answer with WHY ?