You do a Grignard reaction followed by an oxidation.

Step 1: benzaldehyde + phenylmagnesium bromide → diphenylmethanol

Step 2: diphenylmethanol + chromyl chloride → benzophenone

You can use any reagent that converts a secondary alcohol to a ketone.

These include acidified potassium dichromate, pyridinium chlorochromate, and chromyl chloride.