Answer: c. 7,999,999 phone numbers
A can be any digit 2-9, then there are 8 possible numbers.
B can be any digit 0-9, then there are 10 possible numbers.
C can be any digit 0-9, then there are 10 possible numbers.
X can be any digit 0-9, then there are 10 possible numbers.
X can be any digit 0-9, then there are 10 possible numbers.
X can be any digit 0-9, then there are 10 possible numbers.
X can be any digit 0-9, then there are 10 possible numbers.
Then are possible: 8·10·10·10·10·10·10 = 8·10⁶ = 8,000,000 phone numbers
But the number 867-5309 is not used then 8,000,000 - 1 = 7,999,999 possible phone numbers.
Answer: c. 7,999,999 phone numbers