$\frac{x^2}{f\left(4a\right)}+\frac{y^2}{f\left(a^2-5\right)}=1\text{ represents an ellipse with major axis as }\mathrm{y}\text{ -axis and ' }\mathrm{f}\text{ ' is decreasing function, positive for all ‘a’, }$

$\mathrm{then} a \mathrm{belongs} \mathrm{to}$

• A

  (1, 4) 

• B

  (-1, 1)

• C

  (-1, 5)

• D

  $(5,\mathrm{\infty })$