The weather was so plesant in the valley that the hikers decided to extend their stay and explore the surrounding trails for another day.
In the following sentence, four words have been highlighted which may be misspelt or inappropriate in their usage. Mark the erroneous word as the answer and if all the words are correct in their spellings and usage, choose ‘All are correct’ as the correct answer.

The correct answer is plesant.

Let's analyze the sentence and check the spelling and usage of each highlighted word:

1. weather: Used correctly to refer to atmospheric conditions (e.g., temperature, rain, wind). The spelling is correct.

2. plesant: This word is misspelled. The correct spelling is pleasant (meaning enjoyable or agreeable, derived from the verb "please").

3. extend: Used correctly as a verb meaning to make something longer or larger. The spelling is correct.

4. surrounding: Used correctly as an adjective describing the trails in the area nearby. The spelling is correct.

Therefore, the word with the spelling error is plesant.