The correct option is C <76 cm
When elevator goes upwards with some acceleration, the effective value of acceleration due to gravity increases due to pseudo force acting downwards.
⇒g′=g+a ...(i)
When elevator is at rest, the atmospheric pressure measured by the barometer is,
P0=ρgh ...(ii),
where ρ→ density of mercury used in barometer and h=76 cm
When the barometer is kept in an elevator moving upwards with acceleration a, the atmospheric pressure measured by it will be,
P0=ρg′h′
⇒h′=P0ρg′ ...(iii)
From Eq.(i) (ii) & (iii),
⇒h′=ρghρ(g+a)=ghg+a
Since (g+a)>g, h′<h
∴ Reading of barometer will be less than 76 cm