When you have the flu, you may have a cough and a...

Answer: B. running (The correct phrase is "running nose," referring to a condition when mucus flows from the nose, commonly associated with colds or the flu.)

The concert is not... interesting as the one I watched last week.

Answer: D. as (The correct comparison structure is "not as ... as.")

Her favourite food is ... It is a kind of ocean fish.

Answer: B. sausage (Sausage is a common food, and the sentence makes sense in this context. The other options are not appropriate for the given sentence.)

... is your favorite Vietnamese musician?

Answer: B. Who ("Who" is used to ask about a person, which is appropriate for the question about a musician.)