To initiate proceedings for the exercise of the power of the initiative either of the following provisions shall apply as is applicable:
(a) If the petition is signed by duly qualified electors of the city equal in number to at least fifteen percent (15%) of the registered voters of the city, and contains a request that the proposed ordinance be submitted immediately to a vote of the people at a special election, the city council shall either adopt said ordinance within fifteen (15) days after it is presented to the city council by the city clerk, or immediately call a special election at which the ordinance, without alteration, shall be submitted to a vote of the voters of the city.
(b) If the petition is signed by duly qualified electors of the city equal in number to at least ten percent (10%) of the registered voters of the city, and the ordinance petitioned for is not required by Subsection (a) to be, or for any reason is not, submitted to the voters at a special election, and is not adopted without alteration by the city council, then the proposed ordinance, without alteration, shall be submitted by the city council to the voters at the next primary or general municipal election at which said proposition may appear regularly on the ballot or at the next statewide primary or general election into which a special city election may be lawfully consolidated, whichever is the first to occur.
(c) The total number of registered voters of the city shall be determined according to the county clerk's last official report of registration to the Secretary of State.