It involves the binomial theorem to be learned in high school, and the derivation needs mathematical induction. If you are interested, you can learn about it.

Conclusion: (x+1)100 = x100+100x99+4950x98+... So the answer is 4950, choose B.

Without binomial theorem, it may be difficult.

I tried to explain the binomial theorem in a language I can understand since junior high school:

If you don't understand, ask again or write privately.