Which Cloud Strategy do most big companies employ ?

It depends on industry type / volume / revenue. 

e.g:

For SMB / startups, Public cloud is more preferable over Private cloud 

For ecommerce type company - Public cloud is even more suitable options than Private cloud. 

For large sector / BFSI etc Hybrid Cloud with (% Private and% Public)

As per my knowledge it is;

c) Hybrid Cloud

Justification;

Security constraints makes the decision makers to separate the sensitive information and design space to accommodate it on self-managed web-servers.