Hint: The fruit of the cotton plant contains cotton seeds. When it will be fully grown it will produce the cotton and fully mature cotton seeds.
Complete step by step answer: 1) Cotton is a widely grown agricultural plant which has very much economic importance in many countries and widely cultivated across the world for its commercial purpose due to high usage in many forms to human life. 2) The fruit of cotton begins to develop into a green coloured boll from a flower. The immature bolls are a segmented pod containing four segments. There are approximately $32$ immature seeds from which the cotton fibres will grow into fully mature cotton. 3) The boll is considered a fruit because it contains seeds in it which will further develop into fully grown cotton. Individual cells on the surface of the seeds start to elongate from the day the pink flower starts to fall off. 4) The fibres will grow into mature and thicken for the next month, forming a hollow cotton fibre inside the boll which transforms into the approximate size of a small fig. The Bolls grow into its full size in about $25$ days after the petals of flower fall off. After approximately $35$ to $55$ days, the cotton bolls naturally burst into white cotton along with the boll’s segments. 5) These dried carpels are called the bur, and it is the bur that will hold the locks of cotton in the place when they are fully dried, ready for the harvesting of cotton. Hence, the fruit of cotton is called a boll i.e. cotton boll. Note: The fruit of cotton is the green coloured and fluffy boll, not the one white coloured fully grown cotton. One must not confuse one another. The green coloured ball is called fruit because it contains seeds.