The correct option is C 6993
We can make use of distributive property of multiplication over subtraction to make calculations simpler.
This property states that:
a × ( b - c ) = a × b - a × c
7 × 999
= 7 × ( 1000 - 1 )
= 7 ×1000 - 7 × 1
= 7000 - 7
= 6993
Thus, option 'a' is the correct answer.