Step 1: Understanding Basic Minerals 
  
Basic minerals are the essential raw materials used for the production of goods and resources, and they are primarily used in construction, manufacturing, and energy production. 
  
Step 2: Why Iron-ore is the basic mineral 
  
Iron-ore is the most fundamental mineral for the production of iron, which is crucial for construction and manufacturing processes. It is a key component of the steel industry. 
  
Step 3: Why the other options are not basic minerals 
  
- (A) Copper: Copper is an important industrial metal, but it is not classified as a basic mineral like iron.  
- (C) Mica: Mica is a mineral used in various industries, but it is not considered as basic as iron-ore in terms of industrial production.  
- (D) Gold: Gold is a precious metal, not a basic mineral required for broad industrial use. 
  
Thus, the correct answer is option (B) Iron-ore. 
Find the unknown frequency if 24 is the median of the following frequency distribution:
\[\begin{array}{|c|c|c|c|c|c|} \hline \text{Class-interval} & 0-10 & 10-20 & 20-30 & 30-40 & 40-50 \\ \hline \text{Frequency} & 5 & 25 & 25 & \text{$p$} & 7 \\ \hline \end{array}\]