jeans

As opposed to not wearing any pants at all, sure.

You should, however, cover your knees and shoulders in the Duomos throughout Italy.

Jeans are acceptable (cover knees, shoulders and midriff, no t-shirts with obviously objectionable messages) but you may want to wear something lighter during warmer months.

Yes. The only issue is that you wear long pants. And jeans are hot in the summer.

Rome is so hot in summer, and the Vatican is not a/ced,, so ,, wear jeans fine,, but you may roast.

Allowed, yes. Appropriate, maybe. Practical, not at all.