Is it true that only UTXOs can be used as Transaction Inputs? I was reading this GitHub book on Bitcoin

They state that "True or False: Only UTXOs can be used as inputs to transactions."

Is the answer, True because you can only spend what you have. 

Or False, you can out a spent output in the transaction as input, but the network or the miner will reject it.
This question makes no sense.

Of course you can make an invalid transaction that doesn't spend an unspent output. If you're fine with calling invalid things transactions, I could just take a piece of random data and call it a transaction.

A valid transaction can only spend unspent outputs.