Concept:
An idiom is an expression whose meaning is different from the literal meaning of its words.
The idiom
\[
\text{black and blue}
\]
does not mean actual black and blue colours here.
It means badly bruised or beaten severely.
Step 1: Understand the sentence.
The sentence says:
\[
\text{The chain snatcher was beaten black and blue by the mob.}
\]
Here, the chain snatcher was beaten by a crowd.
So the phrase describes the severe beating.
Step 2: Identify the idiomatic meaning.
\[
\text{black and blue}=\text{badly bruised}
\]
or
\[
\text{beaten severely}
\]
Step 3: Check the options.
\[
\text{admired}
\]
means praised, which is not correct.
\[
\text{painted in black and blue colours}
\]
is the literal meaning, not the idiomatic meaning.
\[
\text{arrested}
\]
means taken into police custody, which is not the meaning here.
Step 4: Final answer.
\[
\boxed{\text{thrashed severely}}
\]