I am living in very moisture and humid environment. The eatbles I bought like rusk(made of wheat and vegetable oil), capture moisture within a day and then become unfit to eat. What should I do so that these eatables can stay longer without affecting by moisture and humidity?
I've lived in tropical climates for many years. We stored things like rusks, cookies, biscuits, and the like in air-tight containers. I'd have a large air-tight container for keeping large quantities in which is only opened infrequently, then a smaller one that stores smaller quantities for more frequent access. It's not perfect, but it does mean these products are still good for a longer period of time than if we did nothing.
There is nothing you can do about the food and keep it the same. So you will have to change the environment. The easiest way would be to get a fridge capable of supporting temperatures in the 15 to 20 Celsius range and use it as a pantry. It will be cool, dark and dry, so the ideal pantry environment. If you have the space, you can choose to keep it in your normal fridge or freezer instead.
The one category for which making conserves will work is fruit and vegetables. So instead of storing apricots, you can cook apricot jam. This has the downsides of 1) the end product being very different from the original, and 2) not being applicable to typical pantry items like your rusk. So you still need to find suitable storage space for those. There is no way to simply stop them being affected by the environment.