Explain why iron, cobalt, and nickel are not in the same group of elements

First, remember the distribution of the groups and the periods in the periodic table (see attached file)

The group is observed vertically throughout the table

And the period is observed horizontally throughout the table

As we can see in the table (see attached file), iron, cobalt and nickel are located next to each other horizontally so they belong to the same period and not to the same group

d) the number of neutrons plus the number of protons

t = 1/400 h   ⇔ 0.0025 h ⇔ 9s ⇔ 0.15 m

any of the answers are correct.