The oxidation number of potassium in KNO3 is a) +1.
Explanation: In general, the oxidation state of alkali metals, which include potassium (K), is always +1 when they are in compounds. In KNO3, potassium is paired with nitrate (NO3)-, where nitrogen has an oxidation state of +5 and oxygen has an oxidation state of -2, balancing the overall charge of the nitrate ion to -1. Therefore, to achieve overall neutrality in KNO3, potassium must be +1.