Answer is choice C. The reason why is because we're simply moving parenthesis around to regroup terms
(10)(5x)(7) = (10*5)(x*7)
We've grouped up the 10 and 5 together; we've also grouped up the x and 7 together.
Side Note: The specific rule being used is the associative property of multiplication. Algebraically that rule is a*(b*c) = (a*b)*c