The right answer is 28 years.
The half-life, or period of a radioactive isotope, is the time required for half of the nuclei of this isotope initially present to decay naturally.
One gram of pure 90 strontium has a radioactivity of 5.11 TBq. Its beta radiation is not penetrating, which makes it difficult to measure when it is in the soil or a matrix. It is very harmful when the particle is inhaled or ingested.