Concept:
A synonym is a word with a similar meaning.
The word
\[
\text{Calamity}
\]
means a serious accident, disaster, or event causing great damage or suffering.
Step 1: Understand the meaning of calamity.
A calamity is a very bad event, such as a flood, earthquake, famine, or major accident.
So,
\[
\text{Calamity}=\text{disaster}
\]
Step 2: Check the options.
\[
\text{Calmness}
\]
means peacefulness, which is opposite in sense.
\[
\text{Friendliness}
\]
means friendly behaviour.
\[
\text{Helplessness}
\]
means inability to help oneself, but it is not the synonym of calamity.
Step 3: Final answer.
\[
\boxed{\text{disaster}}
\]