When a square is divided by a diagonal it is ?
a. an isosceles acute triangle is formed
b. an equilateral triangle is formed
c. a scalene obtuse triangle is formed
d. a right isosceles triangle is formed

Let's divide a square by its diagonal and see:

Start with this square:

          

Draw a diagonal:

          

Separate the two triangles:

          

Each of those triangles is a right triangle because a square
has four right angles.

Each of those triangles is isosceles because each has a two 
equal sides because a square has all four of its sides
equal in length.

So since they are both right triangles as well as isosceles,
the answer is (d) although the problem should state 
"TWO right isosceles triangleS ARE formed".

Edwin