To solve this problem, we need to find out how many subjects the student passed the exam in. Let's break down the given information and solve step by step.

Given information:
- The student's aggregate marks in five subjects are in the ratio 10:9:8:7:6.
- The passing marks are 50% of the maximum marks.
- Each subject has the same maximum marks.

Step 1: Calculate the maximum marks in each subject.
Since each subject has the same maximum marks, let's assume the maximum marks for each subject as 'x'.
Therefore, the marks in each subject are 10x, 9x, 8x, 7x, and 6x.

Step 2: Calculate the passing marks.
The passing marks are 50% of the maximum marks.
So, the passing marks are 0.5x.

Step 3: Calculate the aggregate marks.
The aggregate marks are the total marks obtained by the student in all the subjects.
Since the marks are in the ratio 10:9:8:7:6, we can represent them as 10x, 9x, 8x, 7x, and 6x.
Therefore, the aggregate marks are 10x + 9x + 8x + 7x + 6x = 40x.

Step 4: Calculate the percentage aggregate marks.
The percentage aggregate marks are calculated by dividing the aggregate marks by the maximum marks and multiplying by 100.
Percentage aggregate marks = (40x / 5x) * 100 = 800%

Step 5: Calculate the passing percentage.
The passing percentage is 50% of the maximum marks.
Passing percentage = 50%

Step 6: Determine the number of subjects the student passed in.
If the percentage aggregate marks are greater than or equal to the passing percentage, the student passes in that subject.
Let's calculate the percentage aggregate marks for each subject:
- For the first subject (10x): (10x / x) * 100 = 1000%
- For the second subject (9x): (9x / x) * 100 = 900%
- For the third subject (8x): (8x / x) * 100 = 800%
- For the fourth subject (7x): (7x / x) * 100 = 700%
- For the fifth subject (6x): (6x / x) * 100 = 600%

From the above calculations, we can see that the percentage aggregate marks are only equal to or greater than the passing percentage (50%) for the first four subjects. Therefore, the student passed the exam in 4 subjects.

Hence, the correct answer is option 'C' - 4.