I'm just a learner like you, but I'd like to take a stab at it.
Looks like the pairing contains two similar words, with the first word being a stronger version of the second. Zealous also implies something slightly excessive.
with the first word being a stronger version of the second
Thank you! Now I got it. ‘Idolatrous’ IS concerning the excessive devotion to something.
As for the choice C, I just had a thought popped up. Could prudent be a stronger version of cautious like what you said, based on “prude” implying someone who is excessively concerned with app