Let,
Sum of his commission be $S$
Number of sales be $n$
Average be $X$

$X = \frac{S}{n}$

After a $$1000$ commission,
$\frac{S + 1000}{n + 1} = X + 150 = 400$

i.e. $X = 400 - 150 = 250$

So, $S = 250n$ and also $S = 400(n + 1) - 1000$
$250n = 400(n + 1) - 1000$
$250n = 400n + 400 - 1000$
$150n = 600$
$n = 4$

Total sales = $(n + 1) = 5$

Hence, option C