I think it’s second but I’m not quite sure
D. Spring tides <span>Rather, the term is derived from the concept of the tide "springing forth." Spring tides occur twice each lunar month all year long, without regard to the season.</span>
Let the no of months be x
the equation will be-
42+x=3(8+x)
42+x=24+3x
2x=18
x=9
after 9 months Tom will be thrice his dog's age