The expression that represents the phrase "Lamar decreased his collection of cards by seven" is \( x - 7 \).
However, from the options you've given, the closest representation would be \( x - 7 \) if it were provided. Unfortunately, none of the options you've mentioned exactly matches \( x - 7 \). Among the options:
- \( 7x \) (not relevant)
- \( 7 \times 7 \) (not relevant)
- \( 7 - x \) (not relevant)
- \( x + 7 \) (not relevant)
- \( x - 7 \) (would be ideal, but not in your options)
If you specifically meant the expression that indicates a decrease by 7 from an unspecified amount \( x \), the proper option should be \( x - 7 \). Please double-check or reevaluate the options given.