For these types of problems, it's a good idea to think in terms of specific "events". Events occur at specific locations. The instant that Mavis' clock "strikes" 0.400 s is an event and this event occurs at the location of Mavis' timer (that she is carrying with her). We can label this event "E1". Likewise, each specific reading of Stanley's timer is an event located at Stanley's location. The question asks for the time that Stanley reads on his timer when Mavis reads 0.400 s on her timer. This is the same as asking for which event at Stanley's location is simultaneous with event E1 (which occurs at a different location). But Mavis and Stanley are not going to agree in general on the simultaneity of events. In particular, they are not going to agree on which reading of Stanley's clock is simultaneous with Mavis' clock striking 0.400 s.
The wording of the question is, "At the instant mavis reads 0.400 s on her timer, what does Stanley read on his?" I suppose that this could imply that we are asking Stanley to look at his timer at the instant he would consider Mavis' timer as reading 0.400 s. If so, then there is no ambiguity and there is a definite answer. But, it seems to me that the question could also be interpreted as asking for what time Mavis would say that Stanley sees on his timer at the instant her own timer reads 0.400 s. Perhaps most people would take one of these interpretations as being the more natural interpretation.
Anyway, if the question had asked, "At the instant Mavis' timer reads 0.400 s, what does Stanley's timer read?", then it would definitely be ambiguous.