14 m/s or 50km/h. See the details in the attached picture.
The correct option is (D) Gamma (

)
Explanation:Now this question is a tricky one because all of these options are somehow involved in radioactive decay; however, in this case the SYMBOL is required NOT the elements. There are three symbols involved in radioactive decay, which are:
1. α for alpha decay
2. β for beta decay
3. γ for gamma decay
In the options only one symbol is present which is
gamma. Hence option (D) Gamma ( 
) is the correct answer.
-i
We can use the formula,
1/R = 1/r1 + 1/r2 + 1/r3 + ....
Hope this helps!