[D]

A Pythagorean triplets is a triplet which consists of three positive numbers in which the square of one number is equal to the sum of other two numbers such as

a^2+b^2 =c^2

Since,

6^2=36

8^2=64

10^2=100

Therefore,

6^2+8^2=10^2

36+64=100

So the triplet of 6,8,10 is Pythagorean triplets.

[D]

The number of students went on the trip = 361

The number of students went separately = 4

The number of buses filled with student = 7

Therefore, the remaining students who went in a bus are = 361-4 =357

So the number of students in each bus = 357/7 =51

So there are 51 students in each bus.