**Core Concept:**
The question tests the understanding of diagnosing viable pregnancy at 6 weeks based on various methods. In this context, a viable pregnancy is one where the fetus has the potential to survive outside the uterus after birth. At 6 weeks, the key features are the development of foetal heartbeat and the presence of a sacrum.

**Why the Correct Answer is Right:**
The correct answer,**D. Transvaginal Ultrasound**, is the most accurate and safest method to diagnose a viable pregnancy at this stage. Transvaginal ultrasound allows for a closer examination, enabling better visualization of the fetal heartbeat and sacrum, which are the key determinants of viability.

**Why Each Wrong Option is Incorrect:**
A. **Transabdominal Ultrasound**: This method uses a larger probe placed on the abdomen, resulting in less accurate assessment of early gestational age and the possibility of missing vital details like a heartbeat.
B. **Blood Tests**: Although these tests can confirm pregnancy, they do not provide information about the foetal heartbeat or sacrum, making them less reliable for viability assessment.
C. **Physical Examination**: This method is not reliable due to the small size and early stage of the fetus, making it challenging to detect key features like a heartbeat.

**Why the Correct Answer is Right:**
A transvaginal ultrasound allows for a closer examination of the uterus, enabling more detailed evaluation of the pregnancy. Key features such as the presence of a sacrum and fetal heartbeat can be easily detected, making it the most accurate and safe method for diagnosing a viable pregnancy at 6 weeks.

**Core Concept:**
Diagnosing a viable pregnancy in the early stages involves evaluating the development of key features, like the foetal heartbeat and sacrum.

**Why Each Wrong Option is Incorrect:**
A) Transabdominal ultrasound is less accurate due to the probe being placed on the abdomen, making it harder to detect early-stage pregnancy features.
B) Blood tests can confirm pregnancy but do not provide information about heartbeat or sacrum, making them less reliable for viability assessment.
C) Physical examination is impractical due to the small size and early stage of the fetus, making it difficult to identify key features for viability assessment.

**Why the Correct Answer is Right:**
Transvaginal ultrasound allows for a closer examination of the uterus, making it easier to detect key features such as a sacrum and foetal heartbeat. This makes it the most accurate and safe method for diagnosing a viable pregnancy at 6 weeks.