Difference between hold on and hold up

[Pen]

Test No. incompl/advan-32 "Letter Writing (4)", question 5

The result of this was that we were held ......... for six hours in the departure lounge waiting for the end of the dispute.

(a) up
(b) on
(c) by
(d) about

Test No. incompl/advan-32 "Letter Writing (4)", answer 5

The result of this was that we were held up for six hours in the departure lounge waiting for the end of the dispute.

Correct answer: (a) up

Your answer was: incorrect
The result of this was that we were held on for six hours in the departure lounge waiting for the end of the dispute.
_________________________

Why I shouldn't use hold on?

[Alan]

Hold up means delay as it is used here, hold on is hold something firmly. You hold on to your hat in a strong wind.
_________________
English as a Second Language
You can read my ESL story Present Simple

[Rich7]

Yes, you're correct Alan though sometimes "hold on" can also mean to wait as we telephone operator put customers on hold while checking some information. We would say: could you please hold on just a minute?