The four sides of a _________are equal.
a) Square
b) Rectangle
c) Triangle
d) None of these

Hint: For solving this problem, we must check all the options individually. The question demands a geometrical figure whose four sides are equal. So, we consider each option one by one to obtain the answer.
Complete step-by-step answer:
Mathematics includes the study of topics which are related to quantity, structure, space and change. One of the most important branches of mathematics is geometry. It includes the analysis of figures using simple theorems and results.
According to the problem statement, we are required to find the geometrical figure having four sides and particularly all the sides are equal.
Considering option (a), square is a figure of four sides. Also, one of the important properties of squares is that all the sides are equal. Hence it is the correct option.
Considering option (b), the rectangle is a figure of four sides. But the rectangle has two sides equal. Hence, it is incorrect.
Considering option (c), the triangle is a figure of three sides. Hence, it is incorrect.
Option (d) is invalid because we obtain one correct option.
Therefore, only option (a) is correct.
Note: This problem can be alternatively solved by constructing the geometrical figures given in the options. By analysing each figure, we can obtain the correct answer. But knowledge of geometrical figures is required for this method.
Option (a):

Option (b):

Option (c):