Referendums and opinion polls are the two best ways given in the options. But, which is more effective?
An opinion poll is simply, an attempt to measure the opinions of the public of a certain question by questioning a representative sample of the population. Note that key phrase is representative sample. In lay man terms, it is an activity in which many people are asked the same questions in order to find out what most people think about something.
A referendum is a direct vote in which an entire electorate (all the people in a country or area who are entitled to vote in an election) is invited to vote on a particular proposal. This may result in the adoption of a new law. In some countries, it is synonymous with a plebiscite or a vote on a ballot question.
Opinion polls have some disadvantages. First, because they only interview a sample, they are subject to a margin of error. Although in a properly conducted poll this margin should be small, and for almost all purposes for which the polls are legitimately used such a level of accuracy is quite sufficient. However, they are clearly not a substitute for a democratic vote when the electorate is entitled to expect an exact result.
Referendums, on the other hand, involve the entire electorate and not a sample that might turn out to not be representative enough. Also, there are usually campaigns in referendums as in conventional democratic elections and the public have leisure to consider the arguments before making up their minds.
This should tell us who the winner is.