Which sentence is punctuated correctly?
in first-period biology class we learned that, the ecosystem, is a fragile and sensitive thing.
the freshmen, were excited to hear about extra-credit opportunities in their algebra class.
getting her license proved to be more difficult and tricky, than she thought it would be.
in order to serve students best, school districts need to ask for additional funding.

The correct answer is D. In order to serve students best, school districts need to ask for additional funding.

Explanation:

As any other punctuation mark, the use of the comma is guided by strict grammatical rules, most common uses of comma include using comma to separate elements of the same importance in a list; using comma to mark non-essential information and using a comma after introductory phrases. On the other hand, the comma should never be placed to separate the subject and the verb, the verb from the object or any other element in a simple sentence that go togeter..

Considering this, the sentence that is punctuated correctly is "In order to serve students best, school districts need to ask for additional funding" because in this sentence the comma is used to mark the end of an introductory phrase which is correct in terms of grammar. Additionally, other sentences are not correct as the comma separates elements such as the subject and verb or to separate other elements that go together.

the correct answer would be:  

a. adverb phrase