I have corrected and commented where necessary.
a. It’s sometimes difficult to work with someone who finds fault with other colleagues. GOOD but present simple
b. It’s not motivating, according to some people, to work with someone who finds faults in their own work time and again.
This is literal and not the same as the idiom, which is about being critical of other people or other things
c. You shouldn’t find fault with what you don’t understand. GOOD
d. If they keep finding fault with each other, they won’t get ahead.
First conditional. Note "with each other" (not "among").
To make light of: the nearest you got to the sense was b. You should go to the doctor. You don’t have to make light of your health problem.
You "make light" of a problem, a challenge, or an embarrassing situation, so "studies" is not something you "make light of". You might make light of your problems with your studies (I'm doing OK, don't worry about me). However, if you don't take your studies seriously, you "treat them lightly", which is a bad thing. I think you were mixing up the meaning of these two idioms.
Here are some examples of "make light of". https://fraze.it/n_search.jsp?q=make+light+of&l=0
Well done for trying out these idioms - it's not easy to get the true sense of them without regular exposure to conversation. But your approach is a good one if you are working by yourself.