Computer Applications & Bioinformatics

A polymorphic virus is a complex malicious computer program that changes its binary signature and code appearance every time it infects a new system or file, typically using different encryption keys and decryptor routines.

Why Option 2 is correct: Polymorphic viruses are specifically engineered to change their appearance (underlying code and decryption routine) with each iteration while preserving their core functionality, thereby evading signature-based antivirus scanners.
Other options: Spreading across platforms or deleting files are general malware activities, but changing code appearance is the defining characteristic of polymorphism.

Official NTA Answer Key & Analysis:
Official Answer Key: Option 2 (Multiprocessing) is marked in the official NTA answer key.
Technical & Academic Clarification: In formal operating system taxonomy, running multiple programs concurrently on a single CPU by interleaving execution is termed Multiprogramming (or multitasking/time-sharing), whereas Multiprocessing strictly denotes systems utilizing two or more physical CPUs. However, following the official NTA answer key for this question, Option 2 is the official answer.

In Microsoft Excel, every formula must strictly begin with an equal sign (=).
The equal sign signals to Excel that the succeeding characters constitute a formula to be computed rather than plain static text or numerical values.