The word within the question is Dauntless which suggests to point out fearlessness or determination; therefore the antonym of this word would mean something like not fearless or fearful. allow us to undergo the definition of every option one by one.
Cautious means someone who is incredibly careful to avoid potential problems or dangers. This word matches with the desired definition of the antonym of Dauntless. Hence option ‘a’ is that the correct option.
Thoughtful means absorbed in or involving thought or showing consideration for the requirements of others. This has no correlation with the other of the term within the question, therefore is that the wrong option.
Weak means lacking the facility to perform physically demanding tasks; having little physical strength or energy. This word again, just like the previous option isn't an appropriate option and is inaccurate.
Someone is Adventurous if he or she is willing to require risks or to undertake out new methods, ideas, or experiences, with this explanation, options is additionally an incorrect alternative.