The words in the blank must act as the subject of the verb are shipped. Make sure the choice you pick can stand as a complete subject noun phrase before are shipped.

Some choices insert extra verbs (like ships or receive) or an extra pronoun (they) that can make the sentence ungrammatical or clunky. Eliminate any option that creates a confusing or doubled subject-verb structure.

Be wary of options that make the sentence say that a team or a research center is prepared in a greenhouse, or that repeat long phrases unnecessarily.

Strip the sentence down to its core:

• Introductory phrase: Prepared in meticulously regulated greenhouses that replicate the humid understory conditions of tropical forests,
• Main clause (after the blank): _____ are shipped to botanical research centers worldwide for studies on their extraordinary resistance to fungal diseases.

So the blank must be the subject of are shipped and must be something that can logically be “prepared in meticulously regulated greenhouses.”

The phrase Prepared in meticulously regulated greenhouses... should describe whatever comes immediately after the comma.

• If we plug in a choice, ask: Is this the thing that would be prepared in greenhouses?
• A research team or research centers themselves are not prepared in greenhouses; plants (orchid specimens) are.

This lets you eliminate any option where the word right after the comma is not something that could reasonably be prepared in a greenhouse.

Test each wrong option:

• Choice A: Dr. Halstrom's team ships orchid specimens that ...

  • After the comma, the subject is Dr. Halstrom's team, so the sentence implies the team is prepared in greenhouses, which is illogical (dangling modifier).
  • It also produces a wordy structure: ships orchid specimens that are shipped....

• Choice B: botanical research centers worldwide receive orchid specimens that ...

  • Here, botanical research centers worldwide is what’s said to be prepared in greenhouses, which again makes no sense.
  • The phrase botanical research centers worldwide is also unnecessarily repeated later.

• Choice C: fungal disease–resistant orchid specimens, which Dr. Halstrom's team cultivated, they ...

  • fungal disease–resistant orchid specimens is a logical thing to be prepared in greenhouses, but the extra pronoun they creates an error.
  • You get: orchid specimens, which Dr. Halstrom's team cultivated, they are shipped... — this has an unnecessary, ungrammatical second subject (they).

All three have clear structural or logical problems, so they must be eliminated.

The only remaining option is:

• Choice D: the orchid specimens cultivated by Dr. Halstrom's team

Plug it in:

Prepared in meticulously regulated greenhouses that replicate the humid understory conditions of tropical forests, the orchid specimens cultivated by Dr. Halstrom's team are shipped to botanical research centers worldwide for studies on their extraordinary resistance to fungal diseases.

Now:

• The introductory phrase logically describes the orchid specimens (they are what’s prepared in greenhouses).
• The subject the orchid specimens cultivated by Dr. Halstrom's team correctly matches the plural verb are shipped.
• There are no extra pronouns or repeated phrases.

So the correct answer is D) the orchid specimens cultivated by Dr. Halstrom's team.