This is a linear equation.
We shall solve for x using reverse operations.
We know 10 x means 10 times x.
The reverse operation of times is division.
We have
4= 10x
we can also write
10x = 4
Dividing by 10 on both sides we get
x = 4/10
x = 0.4
Hence the value of x = 0.4 makes the statement 4 = 10x true.
Option a) is the right answer.