The middle ages in Europe, saw a sharp decline in mosaics, but what industry during that time, kept it going, at least in a small way...On the floors of churches?

The correct answer to this open question is the following.

Although there are no options attached we can say the following.

The middle ages in Europe saw a sharp decline in mosaics, but the industry during that time that kept it going, at least in a small way, on the floors of churches was the following.

Without any further reference, we could say that it could have been patterned tiles and stone paving. Stained glass played an important role in medieval churches' walls in the Middle Ages.

Traditional Romanesque churches or Gothic churches, both were designed to include fine pieces of art on the floor, in the ceilings, and on the walls. All across Europe, we can see beautiful works of art in the facades and the inside of medieval churches. There are many examples, but we can mention two notorious ones. The Durkheim cathedral and the Canterbury cathedral.

option c (nickel) is not found in earth's crust. the earth's crust basically contains eight (8) elements namely: oxygen, silicon, aluminum, iron, calcium, sodium, potassium and magnesium (listed in order of abundance).