Gprotein-coupled receptors and receptor tyrosine kinases are both largely conserved mechanisms of cell communication. what scientific question is valid to ask when investigating the similarities between these two receptors to understand common mechanisms from which the two may have evolved? how did phosphorylation first begin as a way of modulating protein activity? how early in evolution did cells start using atp-to-adp conversion for energy to transduce signals? how has simple diffusion across a membrane remained a signal transduction mechanism? what factors lead to the ability to use cell energy for dimerization?

The valid scientific question to ask when investigating the similarities between the mechanisms of the two receptors is this: HOW EARLY IN EVOLUTION DID CELL START USING ATP TO ADP CONVERSION FOR ENERGY TO TRANSDUCE SIGNALS?
This question is the right question to ask because, both the G protein coupled receptors and receptor tyrosine kinase use the energy of ATP by mean of phosphorylation to power the movement of different particles through the cell membrane into the cell. This is an example of active transport and during the process ATP is used as a source of energy to move particles across cell membrane. During the process ATP gets converted to ADP.  

Ithink the answer is b, demographic transition theory.