Scientists have been discussing crown shyness since the 1920s, proposing 12 explanations for the phenomenon.
答案：(D) multiple
語意「提出『多種』解釋」，multiple（多種的）符合，呼應後文列舉兩派學者不同的解釋（保護樹枝說、資源競爭說）；universal（普遍的）、productive（多產的）、conventional（傳統的）都不合。
proposing ___ explanations，四個形容詞修飾 explanations 讀起來都不奇怪，所以容易憑印象挑一個看起來比較「學術」的，例如 conventional。這一格考的不是哪個字比較高級，而是後文究竟列出了幾種說法。
往後看：Some researchers point out that...（有些研究者指出樹枝碰撞會斷，所以保持距離）、Other scientists suggest that...（另一些科學家認為植物也會競爭資源）。Some... Other... 這組對照就是答案——不只一種解釋，所以用 multiple（多種的）。最後一句 there is no consensus on exactly what causes...（對確切成因並無共識）更是直接印證：正因為說法有好幾種，才會沒有共識。
universal（普遍的、全體共通的）與「有好幾種不同說法」互相矛盾；conventional（傳統的、慣例的）暗示已經有一個公認的舊說法，但文章說的是尚無共識；productive（多產的、有成效的）拿來形容 explanations 語意不通。
multiple explanations／factors（多種解釋／因素）、multiple-choice question（選擇題）、propose an explanation（提出解釋）、reach a consensus on（就……達成共識）、conventional wisdom（傳統看法、一般認知）。
In certain forests, when you look up you will see a network of cracks formed by gaps between the outermost edges of tree branches. It looks like a precisely engineered jigsaw puzzle, each branch growing just perfectly so it almost, 11 , touches the neighboring tree. This beautiful phenomenon is called crown shyness.
Scientists have been discussing crown shyness since the 1920s, proposing 12 explanations for the phenomenon. Some researchers point out that as trees often grow close together, treetops can easily collide and break when swayed by the wind. In order to protect their branches from breakage, trees maintain "shyness gaps"—spaces large enough to prevent them from touching their neighbors.
Other scientists suggest that plants, like animals, 13 resources—nutrients, water, space, and light. In forested areas with thick tree crowns, there is intense struggle for these resources. Gaps in the treetops resulting from crown shyness may allow trees to increase their 14 light and enhance photosynthesis. Additionally, by having branches that do not touch those of their neighbors, trees may be able to limit the spread of leaf-eating insects, and potentially also the transmission of diseases from tree to tree.
15 decades of investigation, there is no consensus on exactly what causes the beautiful and mysterious phenomenon of crown shyness.