Provided: on the condition or understanding that.
Gained: obtain or secure (something wanted or desirable).
Scattered: occurring or found at intervals or various locations rather than all together.
Invoked: to use a law in order to achieve something, or to mention something in order to explain something or to support your opinion or action.
Filling the blank with 'gained' gives the meaning that such movements have received momentum in recent years.
Other options render the sentence meaningless.
Hence, option (b) is the answer.