WACC

If you have an 8% bank loan then the fact is is described as an 8% loan means that the interest is fixed at 8%. The cost of the bank loan is 8% x (1-T) where T is the tax rate.

Even if it is partly fixed and partly variable? The bank loan that is

As far as the exam is concerned, variable interest would normally apply to short-term finance (overdrafts) which would only be relevant for the WACC calculation if the overdraft was intended to be continued in the long-term. If that were the case then there isn't really a rule. Most sensibly you would try an estimate the long-term average interest rate (probably using the whatever the average was in the past) and use that (after tax relief obviously).