What does Mrs. Starch advise the students to apply before the field trip?

Mrs. Starch advises the students to apply sunblock before the field trip because it serves as a protective layer against harmful UV rays from the sun. During outdoor activities, especially on a field trip, students are often exposed to prolonged sunlight, which can lead to sunburn and long-term skin damage. Applying sunblock helps prevent these negative effects by shielding the skin from excessive sun exposure, ensuring that students can enjoy their trip comfortably and safely.

The other options, while they offer their own benefits, are not the primary concern highlighted in the context of preparing for a field trip in sunny conditions. Moisturizer is typically used for skin hydration rather than sun protection, bug spray can be useful but is more focused on keeping insects away, and insect repellent, while important in environments with many bugs, does not specifically address sun exposure, which is a significant factor during outdoor excursions.