Explanation:

We must not lose heart in times of ______ .

To find the correct word to fill in the blank we will first see the meaning of the given words.

1. diversity -a range of different things.

2. deformity - a deformed part, especially of the body; a malformation.

3. adversity - a difficult or unpleasant situation.

4. inactivity - the state of being inactive; idleness.

After looking at the meanings it is clear that the most suitable word for filling the blank is 'adversity' as it makes the sentence meaningful.

Hence option 3 is the correct answer.