Solution:

The required digit is 0.
If you add up all the digits in any diagonal, you will find the sum to be 25.

Solution:

Flat Tyre

Solution:

Popular Illusion - It can be seen as
1. Vase painted with white
Or
2. Two faces facing each other painted in black.

Solution:

There are twenty-four triangles in the given picture.

Solution:

The rabbit is present in the left tusk of the elephant.

Solution:

36 Quadrilaterals

Solution:

The number missing on the left is 8 and the one on right is 7.

Explanation:
The pattern:
The number in the center of triangle is the square root of the sum of the numbers sitting outside on the corners.

Thus,
10 + 22 + 4 = 36
Square Root of 36 = 6

In the same manner:

10 + 42 + 12 = 64
Square Root of 64 = 8

12 + 22 + 15 = 49
Square Root of 49 = 7

Solution:

It can be achieved as it is shown in the figure.

Solution:

18

Solution:

Word from the first row: AUGUST
Word from the second row: SCRIPT

Word from the circled letters: STRAPS or SPRATS.