Further information will be released when an official date for the ribbon cutting is *blank* next month

Further information will be released when an official date for the ribbon cutting is _____ next month.

This is from the TOEIC questions, I believe. As for the blank, there were 4 choices you would choose from.
A) have planned
B) plan
C) plans
D) planned

The correct answer was C. Could somebody explain why it’s C? (I have lived in the States for 4 years but this one puzzles me. Maybe it’s time for me to study English grammar seriously lol.)

Easy: the correct answer is, in fact, not C. The sentence it produces is not acceptable English:

^*Further information will be released when an official date for the ribbon cutting is plans next month.

Among the answers offered, the best one is D. But even that one,

Further information will be released when an official date for the ribbon cutting is planned next month.

is not that great, either. I would expect something like decided (as Edwin Ashworth suggested), announced, or out.

Discussion

In your sentence, is expects a certain kind of complement. The most obvious one is a past participle, so that with is it forms a passive. An example:

Further information will be released when an official date for the ribbon cutting is announced next month.

The choice planned would fit syntactically, but that is not how one would actually say it.

Another kind of complement that is possible is a predictive complement (PC). That role can be filled by an adjective phrase (AdjP), a noun phrase (NP), or a bare role NP (an NP—but with its determiner removed—which signifies a unique role such as president, captain, treasurer, etc.). Your option C could be interpreted as an NP. However, it doesn't fit semantically: it makes no sense for an official date for the ribbon cutting to be plans next month. Here is one example that would work:

Further information will be released when an official date for the ribbon cutting is a certainty next month.

And one with an AdjP:

Further information will be released when an official date for the ribbon cutting is out next month.

Finally, is can have a complement of location, source, or goal in terms of preposition phrases (PPs). None of the options offered are in this category. Here is an example of how that category might work:

Further information will be released when an official date for the ribbon cutting is in place next month.

Further examples

Here are examples of similar sentences, all from the Corpus of Contemporary American English (COCA):

Dunleavy spokesman Matt Shuckerow said further details will be released when legislation addressing the boards is introduced.

… will be released when it has attained the full, nuanced flavor expected from a straight whiskey.

… for applicants on a staggered alphabet schedule that will be released when sites are announced.

Version 0.4 will be released when WordPress 2.9 is out.

… that is visible only to the operator and will be released when the medium has been changed appropriately.

I'm told he will be released when G'Kar returns home.

Sheikh Obeid will be released when all our prisoners have been released.