Which of these molecules experience london dispersion force?
a. calcium
b. fluorine
c. helium
d. sodium
e. iron

London dispersion forces are the only forces present in noble gases and thus the correct option is helium.  

London force is an intermolecular force which occurs between an atoms or molecules which are polar or non-polar and another atom or molecule which are polar or non-polar. Hence it occurs between all molecules or atoms. The London dispersion force is a temporary attractive force which cause due to temporary dipoles of electrons in two adjacent atoms. This is also known as induced dipole-induced dipole attraction.  It is a weakest Intermolecular force.

therefore the correct option is C.

i'm pretty sure its b i might be wrong though!