Caffeine is a compound found in the seeds of
many different plants, such as coffee beans, cola
nuts, and cacao beans (the source of chocolate).
the presence of this chemical in all three types
of plants suggests that these plants
(1) inherited identical mutations
(2) share a common ancestry
(3) were exposed to the same type of radiation in
the past
(4) were cloned from a caffeine plant

Let's go through the choices. 1) It's impossible for IDENTICAL mutations to have been made through these three different plants otherwise they would be the exact same plant right? So the answer is not 1. 2) A common ancestor would make sense as all three evolved to become different plants but still retain the chemical of caffeine. 3)Woohoo! Radiation is fun, but these plants have been around for much longer than radiation has been around. So the answer isn't 3. 4) The same with 1 and 3, cloning hasn't been around that long and even if it had, clones are the exact replications of the mother plant and would the same plant, not three different kinds. So the answer is not 4. This means the answer must be 2) share a common ancestry.

The answer is observations and interpreting.