C. Kirill loved his trip to the Black Sea because ... 1) he enjoyed the local food. 2) he could swim and relax on the sand. 3) he saw a beautiful sunset every evening. 4) he bought a lot of souvenirs.

Пошаговое решение:

1. Найдем в тексте отрывок, посвященный Сочи и Черному морю: "After Kazan I went to Sochi. The weather there was warm and sunny. I visited the Black Sea and spent time on the beach. I swam in the sea and relaxed on the sand. One evening I watched the sunset, and it was magical."
2. Проанализируем предложения:
3. Вариант 1: "he enjoyed the local food." - О местной еде в Сочи ничего не сказано, только о еде в Казани.
4. Вариант 2: "he could swim and relax on the sand." - В тексте указано: "I swam in the sea and relaxed on the sand." Это является причиной, по которой ему понравилось в Сочи.
5. Вариант 3: "he saw a beautiful sunset every evening." - В тексте сказано: "One evening I watched the sunset, and it was magical." Упоминается только один вечер, и нет информации, что это происходило каждый вечер.
6. Вариант 4: "he bought a lot of souvenirs." - В тексте сказано: "Unfortunately, I didn't have time to buy souvenirs in local shops." Это прямо противоречит утверждению.

Ответ: he could swim and relax on the sand.