Explanation:

Let's break down the problem:

- CAD = 4

- BUS = 4

Let's check if both have 3 letters. If the pattern was "number of letters," the answer would be 3.

But both are said to be 4, so check something else.

Count the alphabetical values:

- C(3) + A(1) + D(4) = 8

- B(2) + U(21) + S(19) = 42

Doesn't fit.

Try number of enclosed spaces:

- CAD: D has 1 enclosed space, so maybe not.

- BUS: B has 2 (top & bottom)

Probably not.

But look at the original data: The answer is 4 for both words, which seem unrelated (length, sum, etc).

Try count of straight lines: Unlikely.

If the answer pattern is just a fixed number, look at options:

- Option 1: 4

- Option 2: 5

- Option 3: 6

- Option 4: 7

But "TRAIN" has 5 letters.

So, final clue: It could be just the number of letters + 1 (as 3 => 4 for CAD & BUS).

So for TRAIN (5 letters): 5 + 1 = 6.

Option 3—6

Option 3 (6)

- The answer is the number of letters in the word plus one.

- CAD (3 letters) ? 4 | BUS (3 letters) ? 4

- TRAIN (5 letters) + 1 = 6

- Correct option: Option 3—6