Answer:
I was on the Editorial Board of the Student's Union monthly magazine in my second year. From the work, I learned the importance of team work and logical thinking.

Explanation:
See i.b. above. Understood is wrong because the second year takes a long time, and learning the importance of team work and logical thinking is a process. In addition, work is not a source.
 

Answer:
In the summer of my second year, I took a two-month training course at Zenith Investment Company, when I got to know many clients personally.

Explanation:
See i.c. above. Two months is a long time, and you can get to know people well.
 

Answer:
From this study, I learned that some of our sales personnel lack proper communication skills.

Explanation:
See i.b. above. A study takes time, and when you study something, you learn things. If the example was 'From this report on your study', you could use 'understood', but a study is not the same as a report on a study.
 

Answer:
I am an active member of the Social Services Group and regularly organise and render various services for the needy. From the work I have done, I have learned to be patient and sympathetic, which will be useful for this job.

Explanation:
See i.b. above. The work is not a source.
 

Answer:
I believe/think I am patient and hardworking and...

Explanation:
See ii above. This is your opinion.
 

Answer:
From your brochure, I understand that you sell a large range of small scale hardware.

Explanation:
See i.a. above. The brochure is the source of the information.