Answer : 4. "सभी (A) और (D)"
Explanation :

Study skills are difficult to define. There is no specific definition of study skills. The wide variations in study methods and habits practiced by learners make it difficult to define and catalog specific study skills. However, in layman's terms we can say that 'study skills' are 'various interests that can be developed to improve learning ability'. It is a process of learning. Good use of study skills can make one an efficient and effective learner.