Concept:The verb must agree with its subject. Two nouns joined by "and" make a plural subject, which takes a plural verb.
Solution:The subject is "Asana and Betty": two people, so it is plural.
"spending their next holidays" talks about a future plan, which English expresses with the present continuous (am/is/are + verb-ing).
The plural present form of "be" is "are": "Where are Asana and Betty spending their next holidays?"
A. "is" and D. "was" are singular, so they do not agree with a plural subject.
C. "were" is past tense, but "next holidays" points to the future.
Key point: In a question the verb comes before the subject, so find the subject ("Asana and Betty") before choosing the verb.
Answer:Option B (are)