The objective to be minimised is:
the constraints on the availability of sufficient funds at the required dates:
positivity: , , ,
The above should be all you need for part (a), part (b) is mixed IP, as is part (c).
The way to handle part (b) is instead of using the amount of the investment use 0-1 variables , and and multiply these by the allowed investment amounts. Then for part (c) you gave an additional constraint: