Since it defines the customer experience, the mobile phone OS is of strategic importance to engineers at Nokia, who assume that if they did not control it, they will not be able to differentiate themselves.
A. if they did not control it, they will not be able to differentiate themselves - Did not is for the Past Tense and Past If cannot have a Future then Part, hence we'll eliminate this option.
B. without controlling it, they would not be able to differentiate themselves - Use of would to represent possibility of a scenario. We will keep this choice.
C .they will not be able to differentiate themselves if they did not control it - Same as A
D. without controlling it, they could not be able to differentiate themselves - Could Refers to an ability someone possessed in the past and no longer has that ability. So we will eliminate this choice.
E. to be able to differentiate themselves would not be possible, had they not control it - Past Perfect had they not control it is not required, as we aren't comparing 2 actions completed in the past.
B is the answer as it's the best solution.