Step 1: Understanding the Concept:
Primary alcohols ($R-CH_{2}OH$) can be oxidized to aldehydes ($R-CHO$) or carboxylic acids ($R-COOH$). To stop the oxidation at the aldehyde stage, a mild oxidizing agent is required.
Step 2: Key Formula or Approach:
Analyze the oxidizing/reducing strength of each given reagent to identify the one that specifically halts oxidation at the aldehyde stage without over-oxidizing the compound.
Step 3: Detailed Explanation:
(A) Alkaline KMnO$_{4}$ is a strong oxidizing agent that oxidizes primary alcohols directly to carboxylic acids.
(B) LiAlH$_{4}$ is a reducing agent, typically used to convert carbonyl compounds back to alcohols.
(C) Na/C$_{2}$H$_{5}$OH is a reducing agent (Bouveault-Blanc reduction).
(D) Pyridinium chlorochromate (PCC) is a specialized mild reagent that selectively oxidizes primary alcohols to aldehydes without further oxidation to carboxylic acids in anhydrous media.
Step 4: Final Answer:
The reagent used for the controlled oxidation of a primary alcohol to an aldehyde is PCC.