Answer - B
for every car accident we can pick a day in $7$ ways
total number of ways in which accidents can be assigned to days $= 7^{7}$
probability of accidents happening on a particular day $=\dfrac{1}{7^{7}}$
we can choose a day in $7$ ways.
hence probability $=\dfrac{7}{7^{7}}=\dfrac{1}{7^{6}}.$