Select the option that expresses the given sentence in passive voice.

We are organising the charity function tomorrow.

To convert the given active voice sentence into the passive voice, let us break down the sentence structure step-by-step:

Active Voice Sentence: "We (Subject) + are organising (Verb in Present Continuous tense) + the charity function (Object) + tomorrow (Adverbial modifier of time)."

Rules for Active to Passive Conversion:
1. Identify Subject and Object: The subject is "We" and the direct object is "the charity function". In the passive voice, the object of the active sentence becomes the new subject: "The charity function".
2. Determine the Tense: The active verb "are organising" is in the Present Continuous Tense.
3. Apply Passive Tense Rule: The structure of the present continuous tense in the passive voice is:
[New Subject] + am/is/are + being + past participle of the main verb (V3).
- "The charity function" is singular, so we use the auxiliary verb "is".
- We add the continuous helper "being".
- The past participle of "organise" is "organised".
This gives us: "is being organised".
4. Assemble the Sentence: Putting the components together along with the time modifier "tomorrow" gives us: "The charity function is being organised tomorrow." (Note that the agent "by us" is optional and omitted here, which is common when the agent is obvious or not the main focus).

Evaluating the Options:
- "The charity function is been organised tomorrow." – Incorrect auxiliary ("been" is used in perfect tenses, not continuous).
- "The charity function is being organised tomorrow." – Correct. It follows the present continuous passive structure perfectly.
- "The charity function is being organise tomorrow." – Incorrect verb form ("organise" is in the base form instead of the past participle "organised").
- "The charity function is organised tomorrow." – Incorrect tense (this is simple present passive instead of present continuous passive).

Therefore, the correct option is: The charity function is being organised tomorrow.