Answer: b. 12.9 grams
Explanation:

According to avogadro's law, 1 mole of every gas occupies 22.4 Liters at STP ,contains avogadro's number
of particles and weighs equal to the molecular mass.
Thus at STP :
1 mole of hydrogen is produced by 2 moles of sodium metal
i.e. 22.4 L of
is produced by 46 gram of sodium metal
6.3 L of
is produced by=
of sodium metal