How many people attended this year?

Statement 1
70 people sent in deposits to attend the Rose seminar this year.
There is no information about the attendance and we cannot rule out the possibility of people that pay deposit but didn't attend.
Hence Statement 1 is INSUFFICIENT.

Statement 2
60% of the people who sent deposit to attend Rose seminar this year actually went.
$$Let\ total\ no\ of\ people\ who\ sent\ deposit\ =x$$
$$no\ of\ people\ in\ attendance\ =60\%\ of\ x\ $$
$$\frac{60}{100}\ of\ x\ $$
There is no definite value for x.
Hence statement 2 is INSUFFICIENT.

Both statement together
From statement 1, we know that , total no of people that paid deposits to attend Rose seminar = 70
$$\frac{60}{100}\cdot70=42$$

Both statements together are SUFFICIENT.
$$answer\ is\ Option\ C$$