________ are those which are used extensively and meet a marked portion of our energy requirement.

The correct answer is: Conventional sources of energy

• Sources of Energy: A source of energy is that which is capable of providing enough useful energy at a steady rate over a long period of time.
• Source of energy are also classified as
  
• Conventional sources of energy and
• Non- conventional sources of energy.
• (i) Conventional Sources of Energy are those which are used extensively and meet a marked portion of our energy requirement and these are :
  (a) Fossil fuels (coal, oil and natural gas) and
• (ii) Non-conventional Sources of Energy are those which are not used as extensively as the conventional ones and meet our energy requirement only on a limited scale. Solar energy, ocean energy, (tidal energy, wave energy, ocean thermal energy, OTE), Geothermal energy and nuclear energy belong to this category.