How many sides does a Decagon have?
$
  (a){\text{ 7}} \\
  (b){\text{ 8}} \\
  (c){\text{ 9}} \\
  (d){\text{ 10}} \\
$

Hint: In this question we have to comment upon the number of sides of decagon. Use the concept that a tri means 3, similar poly means many thus the answer of this problem is hidden in the name of the entity whose number of sides is to be commented upon.

Complete step-by-step answer:

In geometry a Decagon is a ten-sided polygon or 10-gon as shown in figure.
A regular decagon has all sides of equal length and each internal angle will always be equal to ${144^0}$ as shown in figure.
So the Decagon has 10 sides.
So, this is the required answer.
Hence option (D) is correct.

Note: Whenever we face such a type of problem the key concept is simply to have the understanding of the basic geometrical representation of the entity about which it is being asked. The English to mathematical meaning interpretation of the name is always helpful in finding out the sides of the given entity of the question. This concept will help you get on the right track to reach the answer.