Fill in the blanks using the best option:"My mom needed an explanation of all the costs .................. in buying a new cellphone."

To fill in the blank in the sentence "My mom needed an explanation of all the costs .................. in buying a new cellphone," the best option would be "involved."

So the complete sentence would be: "My mom needed an explanation of all the costs involved in buying a new cellphone."

In this context, "involved" refers to the expenses that are part of the process of purchasing a cellphone, such as the price of the phone itself, taxes, fees, and any additional costs that may arise during the transaction. This choice effectively conveys that the speaker's mother is seeking a comprehensive understanding of the total financial implications associated with the purchase.