During project execution, a line manager informs you that he will not have sufficient resources available next month to support you and two others?

Reaction to given situation depends on the " Enterprise Environmental Factors " of an organization. But most common approach is as mentioned in option B .

B. Ask the line manager to set up a meeting with you and the other project managers to work out the problem yourselves.

Thank you for the invitation

Option B

Thank you for invitation. I seem option D. Ask your sponsor to meet with the line manager. is good answer.

My answer is for option (D)

My answer is Option D. Ask your sponsor to meet with the line Manager.

I would like to arrange meeting with line manager, after discussion our priority will be reach to achieve targets within available sources if not this situation we will arrange meet with seniors to get their value able input for success.

Thank you for the invitation

I agree the option : E , Ask the line manager to work with senior management to prioritize the work

Option D can be correct because:

You are countable to sponsor. if you participate to any meeting independently and you committed to some decisions of meeting that they have had negative impact on your project, sponsor will never accept your reasons for negative impacts.

Because sponsors are reponsible for project funding and he is accountable to Portfolio manager(senior manage) then that manager line have to inform his senior manager hierarchically the senior manager to specify the strategic decision 

Option E, line manager must work with program manager (senior management) to provide shared resources until the allocated manpower is not back to work.

Not being agree with option D as sponsor meeting with line manager may initiate process of hiring outsourced team which might be a time consuming activity resulting delay in project delivery schedule