What do you predict would happen if you created a trna with an anticodon of 5′-caa-3′ that is charged with methionine, and added this modified trna to a cell-free translation system that has all the normal components required for translating rnas?

Methionine would replace leucine in the polypeptide at positions encoded by mRNA codon 5'-UUA/G-3'.

Explanation:

The tRNA with anticodon 5'- CAA -3' would bind to mRNA codon 5'-UUA/G-3'. The genetic code 5'- UUA-3' and 5'-UUG-3' code for amino acid leucine.

If the tRNA with anticodon 5'-CAA-3' is charged with amino acid methionine; the tRNA would incorporate methionine in the polypeptide at the position corresponding to the mRNA codon 5'-UUA/G-3'.  

So, methionine would replace leucine in the newly formed protein at some places where respective mRNA had codon 5'-UUA/G-3'.

"d.) carbon dioxide and hydrogen sulfide"

the scientific method.