Formula of the Chloride of an element $M$ is ${\text{MCl}}_4$ and its Molecular weight is $154$ ($\text{Cl}=35.5$). The molecular weight of its oxide is

Given the formula of chloride is ${\text{MCl}}_4$ and its molecular weight is $154$.
Let the atomic weight of element $M$ be $x$.

$$\text{Molecular weight of }{\text{MCl}}_4=x+4\times 35.5=154$$

$$x+142=154$$

$$x=154-142=12$$

Since the atomic mass of $M$ is $12$, the element is Carbon ($\text{C}$).
The valency of $M$ is $4$. The formula of its oxide will be ${\text{MO}}_2$ (or ${\text{CO}}_2$).
The molecular weight of its oxide (${\text{MO}}_2$) is:

$$12+2\times 16=12+32=44$$