Choose the option that best expresses the meaning of the italicised part of the sentence.
The number of aboriginal inhabitants in Australia is considerable.
(a)primitive
(b)ancient
(c)unoriginal
(d)irrational

Hint: Try to understand the meaning of the italicized part with context to the sentence. Then, recall the meaning of the options provided and choose the one which means closest to the italicised part and also fits the context.

Complete answer: A Synonym is a word or phrase that means exactly or nearly the same as another word or phrase in the same language, for example, shut is a synonym of close
The italicized word is aboriginal. The word comes from the word "aborigines" which loosely translates to "original inhabitants". (a)’primitive’- It means old fashioned. Just because you are ancient/indigenous does not mean you are primitive. People who use very basic things are called primitive people- who haven't developed with the fast-paced developments in society.
(b)’ancient’- It means people who are indigenous. Since aboriginals are people who are indigenous people of Australia, they are the natives of that place. They are ancient people who have had generations after generations of people living there.
(c)‘unoriginal’- It is wrong because unoriginal is the opposite of original/indigenous.
(d)’ irrational’- It is incorrect because irrational means not making sense/meaning. He was speaking like an irrational person- his words were not making any sense.

Note: When a sentence is given, it is always helpful to relate the word with the other words and the meaning of the sentence. If you are unaware of the word, use the elimination method. Also, students may be confused between options a and b but remember that primitive and ancient are different words and have differences in meaning.