What became a major form of wall decoration during the Romanesque period? Why?

Combining features of Roman and Byzantine buildings and other local traditions, Romanesque architecture exhibits massive quality, thick walls, round arches , sturdy piers , groin vaults , large towers, and symmetrical plans. The art of the period was characterized by a vigorous style in both painting and sculpture.

Explanation:

Drum columns In most parts of Europe, Romanesque columns were massive, as they supported thick upper walls with small windows, and sometimes heavy vaults. The most common method of construction was to build them out of stone cylinders called drums, as in the crypt at Speyer Cathedral.

the answer is   as follows

d

c

b

a

d