To have the receptor actively signalling, you would need the ligand and a G protein bound. You would need to add the ligand and also have some chemical present, such as an analog of GTP or GDP, which would allow G protein to be bound but not to leave the receptor. The whole complex could then be purified and crystallized.
Work Step by Step
This question is showing you the role of the various steps in signalling by a G protein-linked receptor and asking you to think creatively about how to stall the process at the step of G protein binding to the receptor.