英語里表示“在”的時(shí)間介詞：
1)in表示較長(zhǎng)時(shí)間,如世紀(jì)、朝代、時(shí)代、年、季節(jié)、月及一般（非特指）的早、中、晚等.
如 in the 20th century,in the 1950s,in 1989,in summer,in January,in the morning,in one’s life ,in one’s thirties等.
2)on表示具體某一天及其早、中、晚.
如,on a sunny day,on May 1st,on Monday,on New Year’s Day,on a cold night in January,on a fine morning,on Sunday afternoon等.
3)at表示某一時(shí)刻或較短暫的時(shí)間,或泛指圣誕節(jié),復(fù)活節(jié)等.
如at 3:20,at this time of year,at the beginning of,at the end of …,at the age of …,at Christmas,at night,at noon,at this moment等.
題目中a sunny day為具體的某一天,所以選A.on