Answer:
Option C, The substrate for amylase, starch, is not present.
Step-by-step explanation:
All the tubes (except one) were exposed to following three conditions -
a) a buffer of pH 6
b) a temperature of 104ºF (40ºC)
c) presence of starch
The only tube which was exposed to first two conditions but not the third one i.e starch, do not produce glucose because starch (substrate) was not available for enzyme amylase on which it can act upon under favorable conditions.
The reaction will start only when the enzyme meets the starch.