Step 1: Evaluate the assertion.
Colostrum is the first milk secreted after childbirth and is essential for the newborn.
Hence,
\[
\boxed{\text{Assertion (A) is true}.}
\]
Step 2: Evaluate the reason.
Colostrum is rich in antibodies, especially Immunoglobulin A (IgA), which provide passive immunity and protect the newborn from infections.
Thus,
\[
\boxed{\text{Reason (R) is true}.}
\]
Step 3: Relate the assertion and reason.
Since the antibodies present in colostrum provide early protection against infections,
\[
\boxed{\text{Reason correctly explains the Assertion}.}
\]
Therefore,
\[
\boxed{(A)}
\]
is the correct answer.