He found very interesting to ride a horse on the farm.选填it好,因为只有it才能作形式主语或者形式宾语,
本句中主语是He,谓语是found,缺宾语,正好用it作形式宾语,宾语补足语是very interesting,实际宾语是to ride a horse on the farm.
that,this是不能作形式主语或者形式宾语的.
明白了吗?