A relation is a function if every $x$-value maps to exactly one $y$-value (passes the vertical line test).

• I: Cubic-like curve — passes vertical line test ✓
• II: Circle — fails vertical line test ✗
• III: Square root curve — passes vertical line test ✓
• IV: Exponential curve — passes vertical line test ✓

Functions: I, III, and IV only.