Hey Rashmi! The plural of criterion is criteria. For index, both indices and indexes act as the plural form. However, when you consider the Latin root of the word, the plural is indices, but both forms are used. Usually, in a mathematical context, it's common to use indices, especially when you're talking about the power of a number. You can find more information here. https://www.nasdaq.com/articles/indexes-or-indices-whats-the-deal-2016-05-12

Both, indices and indexes are acceptable plural form of the word index. As for the word criterion, it is the singular of criteria. Criteria is for one or more things. Hope this helps! You can book a conversation lesson with me. I would solve your doubts. Feel free to message me.