The lock and key model of enzymes suggests that the enzyme (the "lock") has a specific shape that perfectly fits a specific substrate (the "key"). Therefore, without a diagram or additional information about the shapes of V, X, Y, Z, or A, it's impossible to determine which one would be the most likely substrate based solely on the names provided.
To answer your question accurately, you would need to analyze the diagram that illustrates the enzyme and the potential substrates to see which one fits the enzyme’s active site. If none of the options fit the active site correctly, then option e ("A realistic substrate is not shown in the diagram") could be the correct choice.
If you can provide more specifics or characteristics of the options, I would be happy to help you further!