Concept :

The Sum of any 2 sides should be bigger than 3rd side of the triangle.

Option A:

5.5 cm + 8.5 cm > 7.5 cm

⇒ 14 cm > 7.5 cm  (yes)

 8.5 cm + 7.5 cm > 5.5 cm

⇒ 16 cm > 5.5 cm  (yes)

7.5 cm + 5.5 cm > 8.5 cm

⇒ 13 cm > 8.5 cm  (yes)

The Sum of any 2 sides should be bigger than 3rd side of the triangle. So, the triangle can be formed.

This option is correct

Option B:

2.5 cm + 6.5 cm > 3.5 cm

⇒ 9 cm > 3.5 cm  (yes)

6.5 cm + 3.5 cm > 2.5 cm

⇒ 10 cm > 2.5 cm  (yes)

3.5 cm + 2.5 cm < 6.5 cm

⇒ 6 cm < 6.5 cm  (No)

The Sum of any 2 sides is smaller than 3rd side of the triangle. So, the triangle can't be formed.

Option C:

4.2 cm +  6.2 cm > 1.9 cm

⇒10.4 cm > 1.9 cm   (yes)

6.2 cm + 1.9 cm > 4.2 cm

⇒ 8.1 cm > 4.2 cm   (yes)

4.2 cm + 1.9 cm < 6.2 cm

⇒ 6.1 cm < 6.2 cm  (No)

The Sum of any 2 sides is smaller than 3rd side of the triangle. So, the triangle can't be formed.

Option D: 

3.1 cm + 4.1 cm > 1.0cm

⇒ 7.2 cm > 1.0 cm   (yes)

4.1 cm + 1.0cm > 3.1 cm

⇒ 5.1 cm > 3.1 cm   (yes)

3.1 cm + 1.0 cm = 4.1 cm

⇒ 4.1 cm = 4.1 cm  (No)

The Sum of any 2 sides is smaller than 3rd side of the triangle. So, the triangle can't be formed.

The correct option is 1 i.e. 5.5 cm, 8.5 cm, 7.5 cm