I don't want to discourage you pursuing education but I strongly urge you to consider finding another course of study. If you complete that course, you will have little useful knowledge of electronics. At least for the 21st century.
I know very little about valves so I can't offer you any assistance with regard to those questions.
With regard to question 14, I can see you've worked out the resistor value using ohm's law which is the correct thing to do. Unfortunately you have made a small error. You have assumed that the 12V supply will be dropped across the resistor - but what about the LED itself?
According to the question the LED drops 1.5V, so that leaves 10.5V that will be dropped across the resistor (they are in series). Re-do your calculations with this in mind and you'll come up with the correct answer.
In my opinion your answers to 14, 20, 21, 23 and 25 are wrong.
Compare Q25 with Q14, they are basically the same question.
In my opinion Q4 is ambiguous. Rotary switches often have many positions, but sometimes only have two.
With regard to question 14, I think I mentioned last time that you cannot assume the entire 12V will be dropped across the resistor. 1.5V is dropped across the diode (according to the question) and, since the resistor is in series, that only leaves 10.5V to be dropped across the ressitor.
Do your calculations with this in mind (rearrange Ohm's law to make R the subject) and you'll come up with the correct answer.