Insolation is-
a) The amount of intercepted solar radiation that reaches the upper atmosphere
b) The amount of solar radiation that reflected into the upper atmosphere
c) The amount of solar radiation absorbed at the Earths surface
d) The amount of intercepted solar radiation that reaches the Earths surface

Dear Matt, 

"Insolation is the incoming solar radiation that reaches a planet and
its atmosphere or, by extension, any object exposed to solar rays,
such as a spacecraft or building."
(SOURCE: Wikipedia, Insolation, <http://en.wikipedia.org/wiki/Insolation>). 

That is, (d). 

I hope this answers your question. Please contact me if you need any
further clarification on this answer before you rate it. My search
strategy: looked up "define:insolation" on Google.