Some insects like water strider is able to walk on the surface of water due to 

CONCEPT:

Surface tension: 

• Surface tension is the property by virtue of which liquid tires to minimize their free surface area.
• In a spherical shape, the surface area is minimum and for this reason, the raindrops are spherical.
• Surface tension is measured as the force acting per unit length of an imaginary line drawn on the liquid surface

\(Surface\;tension = \frac{{Force}}{{length}}\)

EXPLANATION:

• Gravitational force is the force exerted by the earth on an object, gravitational force will only act to pull the insect to the ground hence it won't help the insect to walk on the surface. 
• Insects like water striders are able to walk on the surface due to the high surface tension of the water.
• Water striders use surface tension to their advantage through the highly adapted hydrophobic leg and distributed weight. Hence, option 2 is the answer
• Viscosity is the resistive force acting between the layers of the liquid, Viscosity is the resistive acting between the layers of liquid it won't help the insect to walk on the surface. Hence option 3 is wrong 
• Capillarity is the rise in liquid level against gravity, Capillarity doesn't help the insect to walk on the surface of the water. Hence option 4 is wrong 

Additional Information

• Temperature: If temperature increases then the surface tension of a liquid decreases.
• Soluble Impurities: In the case of less soluble impurities, the surface tension decreases. But, for highly soluble impurities in the liquid the surface tension increases.