Desertification, the situation when desert conditions spread into prev

Official Explanation:

According to the sentence, desertification is the spread of desert conditions into previously arable land and is a global ecological problem. The given sentence is flawed in at least two ways. First, the word situation is an unsuitable choice since it refers to something static rather than a process, which desertification is. A second flaw concerns the way in which when is used. Here, the clause seems meant to modify situation, seeming to function as an adjectival modifier, whereas clauses introduced by when are normally adverbial modifiers, and adverbial clauses are not appropriate modifiers of nouns. A clause introduced by when may modify a noun referring to a period of time, but situation is not such a noun.

A As explained above, this answer choice is incorrect.
B The word condition is unsuitable because of its static connotation, given that desertification is a process. The clause introduced by where seems meant as an adjectival (relative) clause modifying condition but fits awkwardly in that role; although where can be taken as a rough equivalent to in which in casual speech, a condition is not a place, so a where-clause cannot appropriately modify the noun condition in formal writing.
C This version is unnecessarily awkward. The word condition is unsuitable because of its static connotation, given that desertification and spreading are processes. It is unclear whether the adjectival modifier spreading … lands modifies desert or condition.
D One flaw here is the redundancy of previously with the past tense was. Overall, because of the vagueness of where, this answer choice does not make clear that desertification involves the replacement of previously arable land with desert. The expression where land was previously arable is an unnecessarily wordy substitute for into previously arable land.
E Correct. This answer choice provides the clearest and most economical version of the sentence. The locution spread … into accurately and concisely characterizes the process of desertification.

The best answer is E.