GSM networks use microwaves of 20 cm in wavelength.
The energy carried by one trillion (1012) photons of this wavelength is closest to ...
a. 10^-16 Joules
b. 10^-15 Joules
c. 10^-14 Joules
d. 10^-13 Joules
e. 10^-12 Joules
E (per photon) = h c/(wavelength)
I assume you know what h anc d are.
Do the numbers and multipy by a trillion.