Please ans the question.

Dear student
Correct answer is c) 1500m
The speed by which both the actors travelled towards each other was

v1 = v2 = 4km/hr = 4000m/hr
Now, by travelling with this speed they will meet in time
t = s / v, 

here

s = total distance travelled = 400m, 

and relative velocity v = v1 + v2
so,

t = 400 / (4000 + 4000) = 0.05hrs

.
Now, during this time the total distance travelled by the dog will be
s' = t x v'

here speed of dog v' = 30km/hr

so, dog has travelled a total distance of
s' = 0.05 x 30 

s' = 1.5km = 1500 m

With regards