Where would the leucine side chain most likely be found in a protein dissolved in water?

Burried in protein

Explanation:

Leucine have a branched hydrocarbon side chain. The side chain of leucine is nonpolar and hydrophobic.

If a protein have to face water, proteins fold in such a way during the tertiary structure that the amino acid with polar characteristics tend to remain on outer surface while amino acid with nonpolar characteristics tend to remain in the core of protein.

So Leucine side chain would be found burried in core of protein.