The stimulus starts with a general claim: it has been difficult to create a database of all the plant species in the scientific record. We are then given two reasons for why this task is so difficult. First, botanists have for a long time been collecting and naming plants without realizing that many had already been named. In other words, many plants have been given more than one name. Second, DNA analysis has revealed that many plant varieties previously thought to be the same species are actually different species. In other words, some plant varieties that should have different names, because they are actually different species, have until now been classified under the same name.

There isn't much of an argument structure here, and there aren't many actual inferences to make. The stimulus makes a general descriptive claim — it's difficult to make a database of all known plant species — and then explains two factors contributing to this situation. While we could point to some very basic, and almost redundant, assumptions — for instance, that the DNA analysis used by the botanists is reliable — given that the question stem asks for which claim can be "most justifiably" rejected, it's better to treat this like a reverse Most Strongly Supported question. We can check each answer choice and look for the one that clearly has evidence against it from the stimulus.