Concept:
This question asks us to match important epistemological and metaphysical ideas with philosophers.
Step 1: Match Forms are absolute.
Plato held that Forms or Ideas are eternal, absolute and perfect.
So:
\[
A \rightarrow III
\]
Step 2: Match To be is to be perceived.
The statement:
\[
\text{To be is to be perceived}
\]
is the famous idealist principle of Berkeley.
So:
\[
B \rightarrow IV
\]
Step 3: Match causes of impressions are unknown.
Hume held that we only know impressions and ideas, but the ultimate causes of impressions are unknown.
So:
\[
C \rightarrow I
\]
Step 4: Match primary qualities.
Locke distinguished between primary and secondary qualities.
According to Locke, primary qualities exist in the objects.
So:
\[
D \rightarrow II
\]
Step 5: Final matching.
The correct matching is:
\[
A-III,\ B-IV,\ C-I,\ D-II
\]
Hence:
\[
\boxed{\text{(D)}}
\]