Hi Abhijeet

One option is to  enable Restricted access under Site admin > Advanced features.

Create activities for each of the courses you want the user to complete  and make the activities visible when a specified quiz grade is met.

I was thinking audiences could be used to enrol users in appropriate course based on grade but audience rules only cover activity grades.

Another possibility is to have two labels that are visible with a specified quiz grade with a link  to the appropriate course. This course would have self enrolment with an enrolment key required. The enrolment key would be included in the label to prevent everyone from self enrolling in the course.

regards