power fluctuations means both increasing and decreasing , so clearly surge suppressor is not right. Line conditioner is the right answer

Key Word here is "COST-EFFECTIVE" Although a Line Conditioner is better in that its elements eliminate any possible damage to electronic gadget and the Surge Suppressor only limits the damage from potentially destructive events you have to go with the most cost-effective as the Line Conditioner is more costly

It's D. Line Conditioner. "Power line conditioners are your best defense against voltage fluctuations, including brownouts, surges and line noise."

Should be D here, a surge protector would be the cheapest to protect against a power SURGE or SPIKE, but for a fluctuation (up and down voltages), a Line conditioner would likely be more cost effective. From Cheryl Schmidt "Line Conditioners An alternative for computer protection is a line conditioner. Line conditioners, sometimes known as power conditioners, are more expensive than surge protectors, but they protect a computer from overvoltages, undervoltages, and adverse noise conditions over electrical lines. A line conditioner monitors AC electricity. If the voltage is too low, the line conditioner boosts voltage to the proper range. If the voltage level is too high, the line conditioner clamps down the voltage and sends the proper amount to the computer."