According to the formula, if a person can do a piece of work in x days, then the work he can do in one day is $\frac{1}{x}$.

Here,

Person A can complete an assignment in 50 days

So, in 1 day Person A can do $\frac{1}{50}$^th part of the assignment

Similarly Person B can complete an assignment in 76 days

So, in 1 day Person A can do $\frac{1}{76}$^th part of the assignment

Hence option B is correct