Four of the following five are alike in a certain way and hence form a group. Which of the following does not belong to the group?

Study the following information carefully and answer the question given below: Seven boxes of different colours - White, Black, Pink, Brown, Purple, Mustard and Olive are kept one above the other, but not necessarily in the same order. Each box is given a different number viz. 201,211,251,291,331,321 and 325, but not necessarily in the same order. Only three boxes are kept between Olive coloured box and box number 321. Only two boxes are kept between Olive coloured box and Black coloured box. Black coloured box is kept somewhere below box number 321. Only one box is kept between Black coloured box and the box number 211. Purple coloured box is kept immediately below the box number 325. Purple coloured box is kept at one of the positions above box number 321. There is only one box between Purple coloured box and the box the number given to which is less than that given to Purple coloured box. The number given to Purple coloured box is neither 291 nor 331. Only two boxes are kept between box number 251 and Mustard coloured box. The difference between Mustard coloured box and the box immediately below it is less than 80. Pink coloured box is not the topmost box. pink coloured box number is not 211. Only two boxes are kept between Pink coloured box and White colour box.