Final answer:
Rejection code 19 is the code that refers to an invalid days supply in the context of processing prescription claims.
Step-by-step explanation:
The rejection code that describes an invalid days supply is B. 19. When processing prescriptions through a pharmacy benefits manager, various rejection codes are used to indicate specific issues with the prescription claims. Code 19 specifically alerts the pharmacist that the number of days supplied entered for the medication is not valid. This could be due to an entry that exceeds the allowable supply limits set by the payer, or it may be outside the usual and customary supply for the medication prescribed.