The English in this question isn't correct (I am a native speaker). It should say: Samuel received Rs. 10 on the first day of his job. For every day after that, he received Rs. 5 more than the previous day. How much money had he received in total after the 15th day? (my main corrections are: 'after that, he used to receive...' - No, that doesn't make sense, it isn't what you're trying to say. Then "how much did he receive till the 15th day" - that also isn't right and isn't clear. We don't really use "until" (or till) unless we are talking about something changing after that. My wording above is how it should be worded.
Maam, according to me, the correct answer should be atoms as the atoms of noble gases like neon, argon etc have atomicity of 1 so they can exist independently without forming a compound.