When we talk about more than two people/things, 'among' should be used instead of 'between' as it is used when talking about two people/things. This is why the given phrase and option a) are incorrect. And the 'superlative degree' is used to choose one out of a group or many. So, 'the eldest among' is the correct replacement here as 'eldest' is the superlative degree adjective. This is why options b) and d) are incorrect because 'older' and 'elder' are comparative degree adjectives, not superlatives.
Hence, option (c) is the correct choice.
Note:
· The Positive Degree of an Adjective is the Adjective in its simple form. It is used to denote the mere existence of some quality of what we speak about. It is used when no comparison is made.
· The Comparative degree of an adjective denotes a higher degree of the quality than the positive and is used when two things are compared.
· The Superlative degree indicates the highest degree of the quality and is used when more than two things (or sets of things) are compared.
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