The correct option is D are invited
A sentence is in active voice when the subject is the doer of the action.
A sentence is in passive voice when the new subject (object in active voice) is the receiver of that action.
The
given sentence "Effective solutions to the problem..." is in passive voice, as the subject "Effective solutions to the problem" is the receiver of an action. (The subject here is a noun phrase.)
Option A: 'Are invited' is
in passive voice. It indicates that the subject, "Effective solutions"
receives the action 'are invited'. Hence option A is correct.
(The action in the context of the notice 'to be invited' means 'a polite
request is made that the members attending the meeting give effective
solutions to the problem'.)
Option B: 'Have invited' is in active voice. It indicates that the subject performs the action. But here, the subject cannot perform the action 'to invite'. Hence option B is incorrect.
Option C: "Will be invited" is in the future tense, but it is not relevant to the context of the notice. It refers to a person, whereas the subject here is an abstract noun. In comparison to 'are invited', 'will be invited' does not refer to 'the polite request made for some thing', but 'a request made to call someone to a place', like 'invited for dinner'. Hence option C is incorrect.
Option D: 'Are being invited' is in the present continuous tense. It also refers to a person who is being invited at the moment. But this does not provide the relevant meaning, there is no person mentioned in the sentence. Hence D is incorrect.