* They are a member of the legislative body. In most jurisdictions, only members of the legislative body have the power to propose bills. This is because they are the ones who are responsible for making laws.
* They have the support of a certain number of other members. In some jurisdictions, a bill can only be proposed if it has the support of a certain number of other members of the legislative body. This is done to ensure that only bills that have a chance of passing are actually proposed.
* They are an expert in the field. In some cases, a bill may be proposed by an expert in the field who is not a member of the legislative body. This is done so that the legislative body can benefit from the expert's knowledge and experience when considering the bill.
Ultimately, the right to propose a bill is a matter of law. The specific requirements vary from jurisdiction to jurisdiction.