If you invest Rs 1000, you would get a paid back of Rs 4000 at the end of the day, provided atleast 90% of the students invested. If the number of students investing falls below 90%, all those invested will lose their money. Those who decide not to invest will neither lose nor gain anything.
Q What is the optimum strategy for any individual investing in the game? Q What is the nash equilibrium for the game?
It's not clear to me what you mean by "the optimum strategy for any individual".
The question about "the nash equilibrium for the game" seems to assume that there is exactly one Nash equilibrium. This is not the case. Both the strategy profile in which no one invests and the strategy profile in which everyone invests are pure Nash equilibria (if there is more than one student). It is straightforward to see that in both cases the individuals can only lose by individually changing their strategy.