The correct option is B till
A subordinate clause ("the bell rings") depends on the main clause ("You can read your book") for its full meaning. To combine these as a sentence, a subordinating conjunction is required. The subordinate clause here is time-related, the person can read the book for some amount of time before the bell rings. The underlined word 'since' also relates to time, but it refers to an action that began in the past and is continuing in the present, whereas the given statement suggests that the action of 'reading' began in the present and will continue in the future. Hence, 'since' isn't the appropriate option, and D is incorrect. 'Till' is used for time-related sentences, it means 'to the time when', and is relevant to the sentence because the sentence also suggests 'to the time when the bell rings'. Hence option C is correct. 'For' is a coordinating conjunction used when both the sentences in a statement are independent on their own and do not rely on each other to complete their meaning. The given sentence has a dependent and an independent clause, hence a coordinating conjunction will not be used and option A is incorrect. 'Although' is a conjunction used for contrast-related sentences, where the clauses are opposite or contrast in meaning, example, "You can finish your test although the bell has rung". Since the given statement is not contrast in meaning, option B is incorrect.