请教一pp道语法题

mct

lactose, a sugar present in milk, is one of simple sugars used in food prepairation for infants
答案是c.  我觉得b好像有问题。 present 在这里是什么词性呀？

chenchunqiu06

present can be used as adj. and  here it is.
c should be replaced with( of the simple)