There needed to be a way to distinguish the wanted answer (GPCR) from a general transmembrane receptor. There are multiple types of transmembrane receptors that transfer "a signal from outside the cell to the inside of the cell."
"Membrane receptors can be divided into several major groups (Fig. 1-7): (1) seven transmembrane domain G protein–coupled receptors (GPCRs), (2) tyrosine kinase receptors, (3) cytokine receptors, and (4) transforming growth factor-beta (TGF-β) family serine kinase receptors." J. Larry Jameson, in Endocrinology: Adult and Pediatric (Seventh Edition), 2016
A good example is the insulin receptor which is a tyrosine kinase, which does exactly what c2 says, but is not a gpcr.
Adding "7 transmembrane domains" makes the answer non ambiguous.