Step 1: Understanding what 'cool colour' means
Colours are categorized into two main types — warm and cool.
Cool colours are those that remind us of calm, peaceful things like water and sky.
Step 2: Identifying cool colours
Cool colours include Blue, Green, and Purple.
They tend to have a calming effect and are often used in design to create a relaxed feeling.
Step 3: Evaluating each option
(A) Blue $\Rightarrow$ Blue is a classic example of a cool colour.
(B) Yellow $\Rightarrow$ Yellow is a warm colour, associated with sunshine and warmth.
(C) Black $\Rightarrow$ Black is a neutral colour, not specifically warm or cool.
(D) None of these $\Rightarrow$ Incorrect, because Blue is a cool colour.
Conclusion:
Hence, Blue is the correct answer as a typical cool colour.
\[
\boxed{\text{Correct Answer: (A) Blue}}
\]
Find the unknown frequency if 24 is the median of the following frequency distribution:
\[\begin{array}{|c|c|c|c|c|c|} \hline \text{Class-interval} & 0-10 & 10-20 & 20-30 & 30-40 & 40-50 \\ \hline \text{Frequency} & 5 & 25 & 25 & \text{$p$} & 7 \\ \hline \end{array}\]
Two concentric circles are of radii $8\ \text{cm}$ and $5\ \text{cm}$. Find the length of the chord of the larger circle which touches (is tangent to) the smaller circle.