Bow legs:
- Bow legs, also known as genu varum, is a condition where the legs curve outward at the knees while the ankles touch each other.
- This deformity can be caused by a variety of factors, including genetic predisposition, rickets (a vitamin D deficiency), or certain medical conditions.
- In bow legs, the gap between the ankles actually decreases rather than increases.

Flat foot:
- Flat foot, also known as pes planus, is a condition where the arches of the feet collapse, causing the entire sole of the foot to come into contact with the ground.
- Flat foot is usually a result of weakened or stretched ligaments and tendons in the foot, and it can be caused by genetics, certain medical conditions, or injury.
- While flat foot can cause difficulty in walking and running, it does not affect the gap between the ankles.

Scoliosis:
- Scoliosis is a condition characterized by an abnormal sideways curvature of the spine.
- It can cause an uneven distribution of weight on the legs, leading to difficulty in walking and running.
- However, scoliosis does not directly affect the gap between the ankles.

Knock knees:
- Knock knees, also known as genu valgum, is a condition where the knees come together while the ankles are apart.
- In knock knees, the gap between the ankles increases as the knees move closer together.
- This deformity can be caused by various factors, such as genetics, developmental issues, or certain medical conditions.
- It can lead to difficulty in walking and running due to the misalignment of the lower limbs.

Therefore, the correct answer is option B - Knock knees. In this deformity, the gap between the ankles goes on increasing, and an individual may face difficulty during walking and running due to the misalignment of the lower limbs.