• Gullible refers to someone who is gullible because he believes anything. The term "gullible" was coined in the early nineteenth century.
• We know what the word gullible means and thus where it came from. Now consider its application:
  A gullible old man, for example, gave money to a phoney beggar. We can see here that an innocent or naive old man gave money to a phoney beggar.
• Credible- It refers to someone who is reliable and trustworthy. It differs from assuming to be gullible. As a result, it is the incorrect option.
• Believable- Someone or something that is acceptable or credible. It's not the same as aiming for the gullible.
  As a result, it is the incorrect option.
• Credulous- This term refers to someone who is immature and relies on others. It's similar to wanting to be gullible. So, it is the correct option.
• Fallible- It refers to someone or something that is flawed or imperfect. It differs in that it is aimed at the gullible. As a result, it is the incorrect option.
   

Hence, option 'C' is the correct answer.