Train A of length $100$ m crosses a pole in $4$ seconds.
Speed of Train A:
$ \frac{100}{4} = 25 $ m/sec
Train B is approaching towards Train A with a speed of $15$ m/sec and crosses it in $9.1$ seconds.
Let the length of Train B be $x$ meters.
Since Train B crosses Train A, the relative speed of Train B with respect to Train A is:
$ 25 + 15 = 40 $ m/sec
Using the formula:
$ \text{Time} = \frac{\text{Total Distance}}{\text{Speed}} $
$ 9.1 = \frac{x + 100}{40} $
Multiplying both sides by $40$:
$ x + 100 = 9.1 \times 40 $
$ x + 100 = 364 $
$ x = 364 - 100 $
$ x = 264 $
Thus, the correct answer is:
$ \boxed{264} $