उत्तर : 2. "Hidden"
व्याख्या :

"Clandestine" refers to something done secretly or in a concealed manner. Therefore, "hidden" is the most appropriate synonym, as it reflects the concealed or secretive nature of clandestine activities. "Open," "public," and "visible" have the opposite meanings and do not convey the sense of secrecy implied by "clandestine."