Step 1: Look for the rate law:
Order comes from experiment, so recall the measured rate laws.
Hydrogen peroxide decomposition: rate $= k[\text{H}_2\text{O}_2]$, order 1.
Hydrogen with iodine: rate $= k[\text{H}_2][\text{I}_2]$, order 2.
Nitric oxide with hydrogen: rate $= k[\text{NO}]^2[\text{H}_2]$, order 3.
Acetaldehyde decomposition: rate $= k[\text{CH}_3\text{CHO}]^{3/2}$, order $1.5$.
Step 2: Pick the odd one:
Three of the four orders are integers. The acetaldehyde reaction is the lone fractional case, caused by a radical chain mechanism with several steps.
Final Answer:
Option (C).
\[ \boxed{\text{CH}_3\text{CHO} \to \text{CH}_4 + \text{CO} \text{ (C)}} \]