In administration field, which one is better, multi-tasking or solo-tasking and why?

Administration itself is a multi tasking job, which includes Accounting, HR activities, Public relations, Govt. formalities, BD, Customer relations / care etc. 

Multitasking is rarer than you think.  It means "doing two things at once".  What actually happens, when people claim they are multitasking, is that they switch frequently between different tasks.  Even when people are multitasking, there is usually a cost.  For example, salespeople driving between appointments often make phone calls.  This does save time, and usually works out well, but it does increase the risk of having an accident.

The optimum frequency for task-switching depends not only on the tasks themselves, but also on the individual(s) undertaking the tasks.   In general (and there are many exceptions) women tend to task-switch more frequently than men.  Neither pattern is intrinsically better than the other.

I personally cannot agree with Multi-tasking as it is close to impossible for you brain to do two things at a time though I have seen often people talking on phone and calculating simultaneously. Whilst this may be possible to some, multi-tasking would make someone disorganized and less focused. 

I would rather prefer an admin to be very efficient and effective.

It depends on the task or project, some tasks will need focus and full attention so solo tasking should be the way of working to make sure that it will be done perfectly, but on day to day routine work multitasking is the key in order to finish all what is needed to be done and will not have a pending job for the next day, proper time management, being organized and work technique is the key to multitask.

Put your attention on what is right in front of you.  I prefer solo-tasking.

We wrongly call ourselves Multi-tasking because we can not determine the scope of two things we are never the same expert at two things. so, one should adopt solo-tasking you are best at while most of the businesses are looking for multi-tasking employees...