Answer:
To determine the minimum number of flip-flops required to operate a BCD seconds counter, we need to consider the maximum count value and calculate the number of bits needed to represent it.
The given BCD seconds counter can count down from 120 to 0 seconds. The maximum count value is 120, which is represented in binary as 1111000. Since this is a three-digit BCD counter, each digit can take on values from 0 to 9, requiring 4 bits to represent each digit.
Therefore, the number of flip-flops required to operate this counter is equal to the number of bits needed to represent the maximum count value, which is 7.
Therefore, the correct answer is D) 7.