I've done my Diploma in CS after X & am looking for a correspondence course between my job which can be easy, which course should I go for?

You should opt for any programming language course (if possible and, if you are interested, prefer for python) online certification can also be done through Coursera. Com, Udemy etc.

You should opt for any programming language course from a private institute, so then you can manage time both to your belongings.