Concept:
Plato believed that the world of Forms or Ideas is the real world.
The physical world is changing and imperfect.
The Forms are eternal, perfect and unchanging.
Step 1: Understand Plato's Forms.
Examples of Forms are:
\[
\text{Beauty itself}
\]
\[
\text{Justice itself}
\]
\[
\text{Good itself}
\]
These are not objects of sense perception.
Step 2: Check sense experience.
Sense experience gives knowledge of the changing physical world.
It cannot give true knowledge of eternal Forms.
So, sense experience is not the correct answer.
Step 3: Check reason.
According to Plato, true knowledge is obtained by reason.
Reason allows the soul to rise above the world of appearances and understand the Forms.
Step 4: Final conclusion.
Therefore, knowledge of Plato's Forms is based upon:
\[
\boxed{\text{Reason}}
\]
Hence:
\[
\boxed{\text{(C)}}
\]