For study,cocentration and memory power mainly 4 mukhi is very good. Or saraswati mala like 1pc 4mukhi,1pc 5mukhi,1pc6mukhi (3 beads combination mala) also very good.
You can go for either option 4ukhi or saraswati mala. You can wear good nepali beads as these beads are not much costly u can go for 3 or more number of beads it will have more effect..
For student, teachers and who are doing in teaching or speech related job for them these beads are more effecive
om namah sivaya