Melati and Isabel started Bye Bye Plastic Bags by organizing beach cleanups and a petition. When these tactics didn’t work, they

A filmed a video about their cause that went viral.
B boycotted businesses that sold plastic bags.
C decided to go on a hunger strike.
D launched a popular hashtag.

To determine which option is correct, we need to analyze the information given. Melati and Isabel started Bye Bye Plastic Bags by organizing beach cleanups and a petition. However, these tactics didn't work.

Option A states that they filmed a video about their cause that went viral. This could be a plausible next step for them, as a viral video has the potential to garner a lot of attention and support for their cause.

Option B suggests that they boycotted businesses that sold plastic bags. While this is a possible strategy, there is no mention of them taking this action in the information provided.

Option C states that they decided to go on a hunger strike. There is no indication in the given information that they chose this method as their next step. It seems unlikely as a hunger strike is generally a more extreme measure.

Option D suggests that they launched a popular hashtag. This could be a plausible next step, as a viral hashtag can generate a lot of awareness and support for a cause.

Based on the information provided, option A (filming a video that went viral) and option D (launching a popular hashtag) are the most likely actions that Melati and Isabel took when their initial tactics didn't work. However, without additional information, it is not possible to definitively determine which option is correct.