What we have is a single-blank text completion question. Let us deploy IMS's four-step technique to find the right answer.

STEP #1 -> STORYBOARD

Let us first read and grasp the story. Clearly, the sentence talks of a person named Della, who happened to be unfazed (undisturbed) when her husband quickly changed his opinion and then again quickly changed it back. The first part of the sentence comprises a blank, and our task is to figure out which word out of the given answer options could be a contextual fit. What we know from the first part is that Della was wont (accustomed) to her husband's nature.

Now that we have understood the story, let us proceed to the second step.

STEP #2 -> RELATE

Let us find the clues we need to understand better. We may ask ourselves 2 questions:

-> What is the blank dealing with?
-> What information do we have about what the blank is dealing with?

So, what is the blank dealing with? Well, the blank has something to do with the nature of Della's husband. In fact, it must have an adjective that qualifies his nature. And what information do we have about his nature? We know from the sentence that he quickly changed his opinion and then again quickly changed it back.

Now that we have got some insight into what the blank is dealing with, let us proceed to the third step.

STEP #3 -> PREDICT

Let us now come up with our own word for the blank. Remember, we have yet to look at the answer options. It is a must to come up with our own word in order that we may quickly find out the right answer. A person who changes his opinion and then again changes it back can be called 'FICKLE'.

So, 'FICKLE' becomes our own word for the blank. Let us now proceed to the final step.

STEP #4 -> ELIMINATE

All answer options that do not closely match with 'FICKLE' can be eliminated.

A. ruddy - NOT A MATCH - Means 'damned'. Can also mean 'rosy' - ELIMINATE
B. indignant - NOT A MATCH - Means 'angered at something wrong' - ELIMINATE
C. loquacious - NOT A MATCH - Means 'very talkative' - ELIMINATE
D. tranquil - NOT A MATCH - Means 'free from disturbance' - ELIMINATE
E. quicksilver - A CLOSE MATCH - Means 'likely to change due to instability' - MARK AND MOVE

Hence, E is the right answer.