Final answer:
The sentence "Guests must be very careful on the icy trails" fits best after sentence 1, which pertains to snowy conditions when guests first arrive, enhancing the reader's understanding of trail conditions.
Step-by-step explanation:
The best place to insert the sentence "Guests must be very careful on the icy trails" would be after sentence 1 of the paragraph. The sentence directly relates to the conditions experienced by guests at the beginning of their stay, specifically to the situation where the ground is often still covered with snow. Inserting it here helps the reader understand the context of the trail conditions and the caution required before discussing other weather-related issues faced later in the seasons. Here's how it should read:
(1) The ground is often still covered with snow when the first guests arrive for their overnight stay. Guests must be very careful on the icy trails. (2) Later, guests may have to contend with heavy rain...