Answer: C. Your new organism at a succulent mouse who was nibbling on some grass.
A food chain can be defined as a linear sequence in which one organism is consumed by another organism so as to sustain it's life. A primary consumer can be defined as the organism which is typically a herbivore which feeds upon the primary producers like plants and other autotrophs.
According to the situation given, a succulent mouse can be considered as primary consumer who was nibbling on some grass.