Step 1:
A perfectly competitive firm cannot influence market price.
Step 2:
The firm accepts prevailing market price determined by market forces.
It can sell any quantity at that price.
Step 3:
Because price remains constant, firm's demand curve becomes:
\[
\boxed{\text{Perfectly Elastic}}
\]
Thus, the reason correctly explains the assertion.
Step 4:
Hence, the correct answer is:
\[
\boxed{\text{(1)}}
\]