This doesn't make sense to complete the argument. The sentence compares the percentage by which a certain investment is declining to the "benchmark" percentage, which has already been defined as the difference between the inflation rate and the rate of return on the most profitable investment. If the inflation rate had changed, we would have to talk about a change in the benchmark itself, not the difference between the benchmark percentage and the change in value of some other investment.

This might be a tempting answer choice, because it's technically true: this investment must be becoming less profitable, because all investments are losing value in this scenario. But because we know this already, this doesn't actually make sense as a completion of this sentence.

We already know all investments are losing value, at least at the rate the most profitable investment is losing value. We want to know something more specific: what does the fact that this investment is losing value at a higher percentage than the benchmark tell us about the investment? This answer choice is too general to provide a logical answer.

This is correct. If the benchmark percentage is defined as the difference between the inflation rate and the rate of return on the most profitable investment, and this investment is losing value at a higher percentage than the benchmark, then — whatever else is true about this investment — it cannot be as profitable as the most profitable investment. Otherwise, it would set the benchmark, and we would have to talk about a change in the benchmark itself, instead of comparing this investment's rate of loss to the benchmark.

If the rate of return on the most profitable investment available has declined, then the "benchmark" percentage from the first sentence would change. But in the second sentence, we are comparing a particular investment's rate of value loss to that benchmark percentage, not discussing a change in the benchmark itself. So this doesn't make sense to complete the sentence.

This doesn't follow logically. If the most profitable investment available has changed, but has the same rate of return as the previous one, then the benchmark will not change; if it has a different rate of return, then the benchmark percentage will change, and we would be talking about a change in the benchmark, not comparing a particular investment's value to the benchmark. So this doesn't work to complete the sentence.