Yes. Employers need to make contingency plans and to do so need to ascertain where the impact of the action will be. Therefore, it is reasonable to ask the question, but employees do not have to answer and the employer should not put pressure on them to do so.
It is worth remembering that many employees will be unsure as to whether to take action and also hopeful that the action will not be necessary. Therefore, they may not decide to take action until the day of the strike.
What about employees on annual leave on a day on which industrial action is taking place?
In the absence of evidence to the contrary, they should be deemed to be on leave and not on strike.
Authorities will need to be careful about agreeing annual leave for employees requesting it just before strike action starts. Authorities may wish to have a policy of refusing requests for leave made just before a period of industrial action.