The triangular numbers are the following: T1 = 1, T2 = 3, T3 = 6, T4 = 10, ...

They are called triangular numbers because you can think of the number as a series of dots which can be arranged into a triangle as shown below:
T1 =1
T2 = 3
T3=6
T4=10

The formula for the nth triangular number is Tn = n(n+1)/2. For example, the fourth triangular number is 10, so T4 = 10.

1953 is a triangular number. Which one is it?

For example, if 1953 happened to be the 4th triangular number (which of course it's not), your answer would be 4.​