In the following question, match up the prefix with the rest of the word. There is only one solution which uses all of the possible options once.
Firstly, knowing the meaning of the words, prior to having the prefixes added, is useful but not critical. What is necessary is a working knowledge of a broad vocabulary and this is only gained by reading a lot and questioning things which are not known.
The question asks for a single answer that includes all the choices once, therefore there may be some red herrings as one prefix might also work with another word but there will be only one perfect solution. Look for the obvious ones to start with and tentatively make connections for those words. Of the words in the question, 'underground' is an easy starting point. There are no other words that would sound remotely right if they were formed by a listed prefix and 'ground'.
Next, look for any debatable ones and see if you can rule something out. 'Organised' could be preceded by 'dis' but also 'un'; the technique I would teach is to check which of the remaining two words could also be started with 'un' or 'dis' and hope that there is only one. Because 'unbelievable' is a word and there are no other options, we are left with 'disorganised' and 'extort'. As I stated before, there isn't any need to know that 'extort' has a meaning of extracting money with menaces. Recognising the word as 'sensible' is enough; thinking about what the prefix might mean is another helpful way of deciding what is reasonable.
The theme of recent examples has been 'vocabulary'. The advice has always been to encourage your child to read as widely as possible and get them to record words they don't know so that they can question you or use a dictionary when they can. Sometimes, examiners may want to see how broad the vocabulary of their potential intake is and they could simply ask for a number of words which use a particular letter string with a specific meaning. Try this example.
The word 'telephone' is made from two words: 'tele', meaning a long way, and 'phone', meaning sound. List three words which use either of these elements, with similar meanings. Do not include plurals nor any word which alters the spelling of 'tele' or 'phone'.
In this question there is a choice of what to go for but lots of words can incorporate the two parts we have. 'Television' is an obvious one; 'Microphone' is another. Perhaps your child is interested in sci-fi and would have heard of 'teleportation' or will be able to come up with something similar - 'xylophone', 'telescope' etc.
There are few techniques that we can help with once a question is asked; what we need to do is ensure your child is able to think creatively about language before they get to the exam hall. Test them with similar wordplay problems. Give them a letter string and see if they can create a word with those letters at the start, in the middle or at the end. Which letters cannot go together? Check that the dictionary that they have available has more than just the meaning, and shows the origin as well. Let them challenge you in return. On a long car journey, instead of letting them play an electronic game, get them to give you five words beginning with a particular group of letters, then let them challenge you in return. If they have the determination to try and win and interest to research words, they are the sort of child a selective school will want.