Which of the following is a valid real constant in exponent form?

This option is correct as we know that a real constant in exponent form has two parts: a mantissa and an exponent. The mantissa must be either an integer or a proper real constant. The mantissa is followed by a letter E or e and the exponent, the exponent must be an integer. As the given option statisfies all these conditions, hence it is a valid real constant in exponent form.