Explanation
Correct answer: D.
Choice D is the best answer. The convention being tested is subject-modifier placement. This choice makes the noun phrase "pine trees and the fungus Tricholoma matsutake" the subject of the sentence and places it immediately after the modifying phrase "forming…association." In doing so, this choice clearly establishes that the pine trees and fungus—and not another noun in the sentence—are forming the networks.
Why the other choices are wrong
Choice A
Choice A is incorrect because it results in a dangling modifier. The placement of the it-cleft "it is" immediately after the modifying phrase illogically and confusingly suggests that "it" is forming extensive networks. Furthermore, it’s not conventional to follow a long introductory modifying element with an it-cleft construction because it results in an unnecessarily wordy and confusing sentence, such as this.
Choice B
Choice B is incorrect because it results in a dangling modifier. The placement of "the transport of nutrients" immediately after the modifying phrase illogically suggests that the transport of nutrients is forming extensive networks.
Choice C
Choice C is incorrect because it results in a dangling modifier. The placement of "nutrients" immediately after the modifying phrase illogically suggests that nutrients are forming extensive networks.
Student discussion
marcosat 1 likes
Only living beings form symbiotic networks. Ie trees.