An opaque box encloses a region of space. we can't tell what is inside the box but we can see that on all faces of the box there is a non-zero electric field pointing inward. what can we conclude about the charge enclosed within the box?

The charge must be negative

Explanation:

Hi!

The direction of the electric field of a charge depends on the sign of the charge, when it is positive, the electric field points outwards the charge but when it is negative, it points towards the charge. (As you can seen on teh attached image)

So, if on the faces of the box, the non-zero elecric field points inwards, it must mean that the total charge inside the box must be negative