**Core Concept**
The question is testing the ability to diagnose a pregnancy-related issue based on ultrasound (USG) findings. The presence of a gestational sac with absent fetal parts is a critical indicator of pregnancy failure, specifically anembryonic pregnancy or blighted ovum.

**Why the Correct Answer is Right**
The absence of fetal parts within a gestational sac indicates that the embryo has not formed or has failed to develop. This can be due to various reasons, including chromosomal abnormalities, hormonal imbalances, or uterine factors. The gestational sac typically forms around 4-5 weeks of gestation, and the presence of a yolk sac and fetal pole (embryo) is expected by 5-6 weeks. In this case, the absence of fetal parts suggests a non-viable pregnancy.

**Why Each Wrong Option is Incorrect**
**Option A:** Not mentioned - this option is missing.
**Option B:** This option is not provided.
**Option C:** This option is not provided.
**Option D:** This option is not provided.

**Clinical Pearl / High-Yield Fact**
In early pregnancy, the presence of a yolk sac within the gestational sac is a good indicator of embryonic viability. However, the absence of a yolk sac does not necessarily indicate a non-viable pregnancy, as it may be a normal variant.

**Correct Answer: D. Anembryonic pregnancy.**