The correct answer is A, La. Let's look at this!
"Mochila es negrea"
If you are referring to the backpack (Mochila) the pronoun is feminine. There's really no explanation further I can give you as to why it's feminine, just the way the Spanish language said it would be feminine,
Also, if you are referring to black (negra) this is feminine because you are referring to the backpack as feminine, therefore La, should be your prounon!
If you don't understand let me know! I hoped this helped! :)
