Look at the activity series of metals here and pick a metal below Mn but above Cr.
https://www.google.com/search?q=activity+series&ie=utf-8&oe=utf-8
The other part of your question makes no sense. Read it and see.
Which metal could you use to reduce Cr3+ ions but not Mn2+ ions?
Which metal could you use to reduce ions but not ions?
Zn
Mg
Ni
Al
3 answers
Ni
Zn