1. We need to convert a primary alcohol $R-CH_2OH$ into an aldehyde $R-CHO$.

2. This is an oxidation reaction, but the oxidizing agent must be mild enough to stop at the aldehyde stage.

3. Let us examine each reagent:

   A: $CrO_3$
   This is a strong oxidizing agent. Under usual conditions, primary alcohols are often oxidized further to carboxylic acids.

   B: PCC (Pyridinium Chlorochromate)
   PCC is a mild oxidizing agent. It oxidizes primary alcohols to aldehydes without further oxidation to carboxylic acids (especially in anhydrous medium).

   C: $KMnO_4$
   This is a strong oxidizing agent and generally oxidizes primary alcohols all the way to carboxylic acids.

   D: $K_2Cr_2O_7$
   Acidified potassium dichromate is also a strong oxidizing agent and usually converts primary alcohols into carboxylic acids.

4. Therefore, the most suitable reagent for:

$R-CH_2OH \rightarrow R-CHO$

is:

$\boxed{\text{PCC}}$

5. Hence, the correct option is B.