The correct answer is (a). There is no particular number of witnesses required to prove a fact. The number of witnesses required will depend on the circumstances of the case. For example, if the fact is something that is not easily disputed, such as the date of a birth, then one witness may be sufficient. However, if the fact is something that is more difficult to prove, such as the identity of a person, then more witnesses may be required.
Option (b) is incorrect because there is no requirement for two witnesses to prove a fact. Option (c) is incorrect because there is no requirement for only one witness to prove a fact. Option (d) is incorrect because there is no other option that is correct.