Of unknown and unadmitted authorship

A. Gullible

B. Anonymous

C. Unanimous

D. Vexation

Answer: Option B

Solution(By Examveda Team)

One word substitute is Anonymous.

Gullible: easily persuaded to believe something; credulous.
Anonymous: (of a person) not identified by name; of unknown name.
Unanimous: (of two or more people) fully in agreement.
Vexation: the state of being annoyed, frustrated, or worried.