Which of the following is not a number expressed in scientific notation?

The correct option is D $10\times {10}^2$
Detailed step-by-step solution:
Scientific notation: $a\times {10}^n$ is said to be in scientific notation if $1\le a<10$, and the exponent n is a positive or negative integer.
Let’s check the options:

A. $5.34\times {10}^9$: The factor $5.34$ lies between $1$ and $10$ and the exponent $9$ is a positive integer. Hence, the number is expressed in scientific notation.

B. $9.1\times {10}^{-2}$: The factor $9.1$ lies between $1$ and $10$ and the exponent $-2$ is a negative integer. Hence, the number is expressed in scientific notation.

C. $9.0\times {10}^3$: The factor $9.0$ lies between $1$ and $10$ and the exponent $3$ is a positive integer. Hence, the number is expressed in scientific notation.

D. $10\times {10}^2$: The factor $a=10$ which does not satisfy the $1\le a<10$. Hence, the number is not expressed in scientific notation.

Therefore, option D is correct.