In the logarithmic equation \( \log_b x = y \), the following restrictions apply to \( x \):
- \( x \) must be positive: The argument of a logarithm must be greater than zero; therefore, \( x > 0 \).
- \( x \) does not have to be even or an integer; it can be any positive real number as long as it meets the criteria.
So the correct response from your options is:
- x must be positive.