A.这句话的主句make a pause where there is a full stop是个祈使句,主语为you,一般省略.
本题的考点是省略.当主句和从句的主语一致时,从句可以采用省略的形式.
B是错误的,将该项中省略的词补充出来为When you are read, make a pause where there is a full stop.当然这是病句.
本题A项中,从句while you read 的主语you和主句的主语you一致,可以省略,故答案也可以是while reading.