Axial thrust is minimum in case of

Explanation:

Helical gear: 

• These gears are used to connect the shafts whose axes parallel and teeth are inclined to the axis.
• In helical gears due to the angle of the teeth, they create a thrust load on the gear when they mesh. 

Double helical or Herringbone gears:

• These gears are used for transmitting power between two parallel shafts.
• They have opposing helical teeth. Two axial thrusts oppose each other and nullify.
• To avoid axial thrust, two helical gears of the opposite hand can be mounted side by side, to cancel the resulting thrust forces. These are called double helical or herringbone gears.

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Spur Gear:

• They have straight teeth parallel to the axes and thus are not subjected to axial thrust due to tooth load while meshing.
• At the time of engagement, the contact extends across the entire width on a line parallel to the axes of rotation. This results in the sudden application of the load, high impact stresses, and excessive noise at high speeds.

Bevel Gear: 

• Bevel gears are used to transmit motion between shafts at various angles to each other.
• The teeth profile may be straight or spiral.