The mean free path of molecules of a certain gas at STP is 1500d ,...

The mean free path is given by the relation,

$\lambda = \dfrac{RT}{\sqrt{2}\pi{d}^{2}{N}_{A}P}$

So, $\lambda ∝ T$

Let $\lambda '$ be the new mean free path.

Hence,

$\dfrac{1500d}{\lambda '} = \dfrac{273}{373}$

$⇒ {\lambda}^{'} = 2049 . 45d \approx 2049d$