Option 1 -> Hidden means concealed or not visible, which is the opposite of conspicuous.
Option 2 -> Obvious means easily seen or noticed, which is a direct synonym of conspicuous.
Option 3 -> Subtle means understated or not obvious, which is the opposite of conspicuous.
Option 4 -> Vulnerable means susceptible to harm, which is unrelated to the meaning of conspicuous.
Hence, Option 2: Obvious -> Conspicuous means standing out or attracting attention, making "obvious" the most similar word in meaning -> correct