I am not sure why the answer asserts that option II is also false. From what I can see on the graph in Figure 1, it looks to me like when the mice are missing the GCAP1 gene, they produce more cGMP than the wild type. If the mice are producing more cGMP without the GCAP1 gene, it doesn't make sense that this gene is necessary for normal photoreceptor recovery.
Thanks in advance.