选B
reach作为到达讲时是及物动词,其后必须跟宾语,如When will they reach Beijiing?他们什么时候到达北京?
而在此句中,here 是副词,在句中做状语,不是宾语,所以不能用reach.只能用arrive,它是不及物动词,后面接宾语时须加介词at或者in.大地方用in,小地方用at.
比如：They will arrive in Beijing at six o'clock.
They will arrive at the school at six o'clock.
此句中here是副词,作状语,arrive后面无须加in或者at,直接用不及物动词arrive加副词即可.
其他例子还有：When will they arrive there?
When will you arrive home?
there,home均为副词.
所以本题选B