A) In the class struggle the workers conquer more rights using strikes, organizing themselves, etc. There are several pages in Marx's writing detailing this happening.
B) Like any other commodity, the labor force could became more expensive, entirely following capitalist rules. This would be the case if the price of food increases: you cannot pay for your workers less than the needed for their subsistence in a sustainable model, which means that increases in the living price requires more wages. But this also could happen in other scenarios, like if the society becomes more complex. If now the workers need more things to properly function just because the society is more complex and they require more things in order to produce more complex labor. Then they would get more wages to buy their computers, TVs, pay for education, etc. As a bonus, now these things can also be sold, helping capital to reproduce itself. Marx says that what the workers need in order to produce their own livelihood, the standard of living of their day, is socially and historically determined. This is not a fixed thing.
What Marx says is not that wages cannot improve in capitalism, but that the surplus value would always increase (except for changes caused by scenario A). Which was a perfectly correct prediction: if you check how worker productivity increased over time and how wages increased, you would notice that there is a gap that is always increasing. Moreover, the workers also could not own the relevant means of production: they always would need to sell their labor force.