$0.5$ g atoms cu= $0.5 \times 6.023 \times 10^{23}$

$\qquad= 3.0115 \times 10^{3}\;cu\;atom$

$0.635\;g cu=\large\frac{0.635}{63.5}$$ \times 6.023 \times 10^{23}$

$\qquad=6.023 \times 10^{21}\;cu\;atoms$

$0.25$ mole $cu=0.25 \times 6.023 \times 10^{23}$

$\qquad= 1.5 \times 10^{23} \;cu\;atom$

$1 \;g Cu =\large\frac{1}{63.5} $$ \times 6.023 \times 10^{23}$

$\qquad= 9.48 \times 10^{21}\;cu\;atom$

Hence a is the correct answer.