Otto's Tune$-$Up Shop follows the revenue recognition principle. Otto services a car on Au$-$gust $31.$ The customer picks up the vehicle on September $1$ and mails the payment to Otto on September $5.$ Otto receives the check in the mail on September $6.$ When should Otto show that the revenue was recognized?

a$.$ August $31$

c$.$ September $5$

b$.$ August $1$

d$.$ September $6$