Hi Em.C11!
I think you would be able to talk about what you want with this question.
You don't have to fully agree/disagree and say murder is/isn't the focus of the film. If I got this question I would say that the murder mystery plot intertwines with a romantic comedy plot that acts as a social commentary on the time period. Since it mentions murder mystery, you should have at least one paragraph about the murder mystery (like how it drives the plot forward /is the main focus). Then you could have a paragraph about how one of Hitchock's aims of this film was to comment on this era and talk about gender roles/McCarthyism/etc in the 1950s. For my last paragraph, I would write about how the film is not just a murder mystery, the main focus/a large part of the film is (the relationship between Jeff and Lisa) OR (voyeurism), and talk about how it's a significant part of the plot.
Hope this helps :)