El oso muy grande y feo.
a. fue
b. eran
c. era

Era is the conjugation of the verb ser that matches the third person singular in the imperfect.  In general, the imperfect is used to talk about past actions, conditions, or events that occurred regularly or frequently or that were in progress in a moment in the past. So here we use the verb ser as physical description. Finally:

El oso era muy grande y feo

when you are going to explain a procedure,   the first thing to do is to name the parts or the apparatus you are going to describe in terms of how it or they function. also it is important to know who is going to operate it and the abilities this man/woman requires to manage it.