selecting correct stain and varnish/poly/something else for oak hardwood floor
Your floors are prefinished and likely have an aluminum oxide finish on them which will be tough to sand off and require more time and sandpaper than normal. It also appears to have microbevels as mentioned by previous posts. You will need to either sand to a consistent depth or completely sand these microbevels off. If you do not sand them off completely there will be residual finish in them which will make applying finish to the floor via T-bar or paint pad difficult. 

Environmental regulations have made polyurethane less desirable and it levels more poorly than it did previously. 

Some of the most durable finishes available for site applied applications include Bona Traffic, Vermont Natural Coatings Polywhey and Basic coatings "Streetshoe". You should choose a finish and then select a stain as recommended by the finish manufacturer. Bona sets up in a few hours and therefore you can seal and do two coats in a day.