Every time I turn on macbook pro I get menu for choosing startup disk Every time when I turn on my macbook pro mid 2014 I have to choose my startup disk to continue with booting. It behaves as if I am holding the option key...

However, the right option key doesnt work when I want to use it in the OS.

When I turn on MP and hold the left option key it starts booting without boot menu, when I do not hold it it shows the menu. 
When I hold the right option key there is no difference. 

I have tried apple diagnostics and it shows that everything is ok. 
Tried resetting SMC and PRAM and doesnt work.
Going out on a limb here due to lack of information, just trying to help.

Your Mac is 4 Years old, and probably (if you are like me) you have a "lunch" stuck under the right Option key.

Clean it.

https://www.ifixit.com/Guide/How+To+Fix+Sticky+Keyboard+Keys+on+a+MacBook/37709

If that did not help you might have to replace the key.