A "reasonable substitute" means finding something that is similar or acceptable enough to take the place of something else. It may not be exactly the same, but it can still serve the purpose or fulfill the need adequately. Full definition
We know you all are going to ask what can be substituted and I'm going to argue nothing... but, if you must violate this recipe the only reasonable substitute for cranberries would be a tart green apple. (realeverything.com)
But with just an HR monitor and accelerometer, it's not exactly any kind of reasonable substitute for a fitness tracker. (gizmodo.com)
For example, children at this age often use realistic objects and / or reasonable substitutes during pretend play (eg they might use a banana as a telephone) (kidsmatter.edu.au)