Answer: a. an express warranty
Step-by-step explanation:
An EXPRESS WARRANTY refers to spoken or written promises or guarantees made by the seller about the performance of a product when negotiating with a buyer.
For example, "this product is guaranteed against wear and tear for at least 2 years".
Roy Mustang in this scenario made an explicit promise about the effectiveness of the Stallion and as such that constitutes an EXPRESS warranty.
It is worthy of note that for Express Warranties, the word Warranty does not even need to be included for the promise to be a warranty.