In the water cycle, which process allows liquid water from the Earth's surface to enter the atmosphere as water vapour?

Introduction / Context:
The water cycle describes how water moves continuously between the oceans, atmosphere and land. One important step is the conversion of liquid water at the surface into water vapour that rises into the atmosphere. This question asks you to identify the main process responsible for that change of state on a global scale.

Given Data / Assumptions:

• We are considering the movement of water from the Earth's surface into the atmosphere.
• Water can change state from liquid to gas under suitable conditions.
• Several biological and physical processes may produce vapour.
• The question focuses on the primary physical process in the water cycle.

Concept / Approach:
Evaporation is the process by which liquid water at the surface changes into water vapour. It occurs from oceans, lakes, rivers, soil and wet surfaces when molecules at the surface gain enough energy to escape into the air. Transpiration, the release of water vapour from plant leaves, also contributes to atmospheric moisture, but at basic level the main physical mechanism described in textbooks for water entering the atmosphere is evaporation. Condensation is the opposite process, where vapour turns back into liquid, forming clouds and rain, and respiration produces relatively small quantities of water vapour compared to evaporation and transpiration combined.

Step-by-Step Solution:
Step 1: Recall the definition of evaporation: the change of water from liquid to vapour at the surface of a liquid, even below the boiling point. Step 2: Note that large water bodies like oceans and lakes are constantly losing water to the atmosphere by evaporation. Step 3: Understand that this vapour rises, cools and later may condense to form clouds and precipitation. Step 4: Transpiration is similar in that plants release vapour, but it operates through living tissues and is usually treated separately as a biological process. Step 5: Since the question asks what process allows water to enter the atmosphere in general, the best single answer is evaporation.

Verification / Alternative check:
Diagrams of the water cycle in school textbooks usually show arrows from oceans and lakes to the atmosphere labeled "evaporation" and arrows from plants labeled "transpiration." Together these are sometimes called evapotranspiration, but evaporation is the main physical phase-change from liquid to gas at surfaces. Condensation is shown from clouds to droplets, which is clearly the reverse process. Respiration contributes a relatively small fraction of atmospheric vapour and is not usually highlighted in basic water cycle diagrams, supporting evaporation as the standard answer.

Why Other Options Are Wrong:
Transpiration from plant leaves: This does add water vapour but is a biological sub-process; the dominant physical process from large water bodies is evaporation.

Respiration in animals and plants: Produces water vapour as a by-product, but its overall contribution is smaller and it is not the primary driver in the water cycle diagrams.
Condensation of water vapour: This is the change of vapour back to liquid, the opposite of what is asked.

Common Pitfalls:
Students sometimes confuse evaporation and condensation because both involve phase changes. A good memory aid is that evaporation starts from a liquid surface and sends water up into the air, while condensation happens in the air and brings water down as droplets or rain. Also remember that plants assist through transpiration, but evaporation from oceans and lakes is the main global contributor.

Final Answer:
Water mainly enters the atmosphere from the surface through Evaporation from water bodies and moist surfaces.