ISO 400 is twice as sensitive and ISO 100 true or false

Quite simply, when you double your ISO speed, you are doubling the brightness of the photo. So, a photo at ISO 400 will be twice brighter than ISO 200, which will be twice brighter than ISO 100.

Explanation:

ISO most often starts at the value of ISO 100. This is the lowest, darkest setting, also called the base ISO. The next full stop, ISO 200, is twice as bright, and ISO 400 is twice as bright than that. Thus, there are two stops between ISO 100 and 400, four stops between 100 and 1600, and so on.