An Effect
Effect
noun, の adjective
The fruit of something that's effective is an effect.
This is a jukugo word, which usually means on'yomi readings from the kanji. If you know the readings of your kanji you'll know how to read this as well.
ジムで水泳をすることが、最も効果的なダイエット方法だと聞きました。
I heard swimming at the gym is the most effective way to lose weight.