Answer:
The answer is indeed letter D. knows; are
Explanation:
When we use "or" to join words that function as the subject in agreement with a verb, we must look at those words to decide if the verb should conjugate in the singular or the plural form. The rule is actually quite simple. If both words are singular, the verb is also singular. If one or both words are plural, then the verb is also plural. Note that it is enough for one of them to be plural.
In "Sean or Stacy", Sean is just one person (singular) and Stacy is just one person as well (singular). Thus, the proper conjugation is singular: Sean or Stacy knows. In "shuttles or other space vehicles", shuttles and vehicles are both plural words (the -s in the end represents their plural form), meaning there are more than just one shuttle and one vehicle. Thus, the proper conjugation is plural: shuttles or other space vehicles are launched.
The full sentence is, therefore:
Sean or Stacy knows when shuttles or other space vehicles are launched.
