Meaning of mode string in fopen: In the call fp = fopen("source.txt", "rb"); what does the mode "rb" instruct the C library to do?

Introduction / Context:The fopen mode string controls how a file is opened—text vs binary, reading vs writing, appending, and whether a new file is created or an existing one is truncated. Choosing the correct mode avoids data loss and platform-specific newline translations.

Given Data / Assumptions:

• Call shown: fopen("source.txt", "rb").
• Standard C fopen modes apply.
• We aim to interpret the two-character mode "rb".

Concept / Approach:

• "r" means open existing file for reading.
• "b" means binary mode (no text translations such as CRLF to LF on Windows).
• To read and write, modes like "rb+" or "r+b" are used; to create/truncate for writing, use "w" variants.

Step-by-Step Solution:

Verification / Alternative check:

Why Other Options Are Wrong:

• Binary read/write: Requires "+" in the mode.
• Create for read/write: That is "wb+" or "w+b", not "rb".
• None of above: Incorrect because option A is accurate.

Common Pitfalls:

• Opening text files in text mode on Windows and being surprised by newline translation; use "b" for binary data.
• Forgetting that "r" fails if the file does not exist.

Final Answer: