Option C represents the appropriate YARA matching approach because the analyst is searching for an encoded representation of ActiveMime , not necessarily the literal plaintext string. The grep -rail ActiveMime * command performs a case-insensitive recursive search for the textual value ActiveMime. If the payload contains that value encoded—commonly through Base64 or another transformed representation—the literal byte sequence will not be present and grep will therefore fail to identify it.

YARA is designed specifically for pattern-based identification of malicious or suspicious content. Its string modifiers support encoded representations. YARA's official documentation states that the base64 modifier searches for Base64-encoded forms of a specified string, while base64wide extends this behavior to wide-character representations. Therefore, a rule constructed to detect the encoded form of ActiveMime can identify relevant documents without requiring the literal plaintext sequence to occur in the file.

The essential examination concept is the distinction between literal text searching and malware-oriented encoded-pattern matching . Rules that search only for ordinary ASCII ActiveMime reproduce the limitation of the unsuccessful grep command.

Study Guide Reference: Security Operations → Malware Analysis → YARA → Signature/Pattern Matching → Encoded Content → Base64/Base64wide → File Analysis.