Hint: The viscosity of a substance, in simple words, can be defined as the resistance of the substance to flowing. So, it can be said that the viscosity of a substance is inversely proportional to its ability to flow. The higher the viscosity, the lesser the ability to flow freely.
Complete step by step solution: To find the substance with the highest viscosity, let us compare the ease with which each substance in the given options can flow.
(A) Wood: Wood is a solid substance. If a piece of wood is kept on a surface without applying any external force on it, then we find that the piece of wood does not move on its own. This simply shows that there is very high resistance to the movement of the wood, and therefore, the viscosity in wood is maximum as it is solid.
(B) Water: Water is a liquid, and therefore, has viscosity. If we pour water from a vessel on the floor, we find that the liquid flows freely on the floor without showing resistance. The great ease with which water flows on a surface shows that it has a low viscosity.
(C) Oxygen: Oxygen is a gas, and hence, possesses viscosity. Gases can move freely and occupy the entire space available to them. Gases can move more freely than liquids. This is possible because there is no resistance to their movement, which shows that the viscosity in gases is minimum. Therefore, the viscosity in oxygen is lower than that in water.
(D) Sauce: The sauce is a liquid, but it is thicker than water. The viscosity of liquids increases with thickness. Since the sauce is thicker than water, its viscosity is higher than that of water. The higher viscosity of sauce is confirmed by the fact that when both sauce and water are poured on a surface, the water flows faster than the sauce, which shows that there is greater resistance to the movement of sauce due to higher viscosity. Therefore, the order of increasing viscosity for the given substances is: Oxygen $<$ Water $<$ Sauce $<$ Wood
So, the correct answer is Option A.
Note: Viscosity is a fluid phenomenon. It occurs only in liquids and gases, and not in solids. Viscosity is low for thinner liquids and high for thicker liquids. Order of viscosity:The general order of viscosity for substances is: Gases $<$ Liquids $<$ Solids