Which of the following explanations is not an explanation for decreased variability of the fetal heart tracing:
Baseline varibility on CTG is the oscillation of baseline FHR excluding acceleration and deceleration.
A varibility of 5-25 BPM is a sign of fetal well-being.
Decreased varibility is seen in:

Fetal hypoxia
Sleep apnoea
Congenital malformation
Maternal academia
Infection
Drugs given to mother such as sedatives, magnesim sulfphate, antihypertensives

Williams 22/e, p 499: Drugs given to moter which decrease the baseline varibility are:

Narcotics
Barbiturates
Phenothiazines
Tranquilizers
General anaesthetics
Meperidine
Butorphanol
MgSO4

According to Wiliams Increasing gestational age leads to increased baseline varibility, so it can be taken prematurity will have decreased varibility (i.e. option ‘b’ can be considered correct).
Fetal "sleep state"
Prematurity
Barbiturate ingestion
Fetal stimulation