Mutual efforts are the key to success. An effort by one person is the key to success.

Efforts = plural. Use "are".

You could say either: "Mutual efforts are the key to success". OR "Mutual effort is the key to success". Efforts are... (plural) Effort is... (singular)

I am the key You are the key He/she/it is the key We are the key You are the key Theye are the key