Which of the following physical layer standards is part of the IEEE 802.3u Fast Ethernet specification?

Introduction / Context:
IEEE 802.3u defines Fast Ethernet, which operates at 100 Mbps. It includes several physical layer standards that describe how 100 Mbps Ethernet can run over different media types, such as twisted pair and fiber. Knowing which specific variants belong to 802.3u helps in selecting the correct cabling and hardware for a network.

Given Data / Assumptions:

IEEE 802.3u is the Fast Ethernet standard, operating at 100 Mbps.
Common Fast Ethernet physical layer types include 100Base-TX, 100Base-T4 and 100Base-FX.
The options list several Base-X styles, some of which correspond to other Ethernet families.

Concept / Approach:
Historically, 10Base5 and 10Base2 are 10 Mbps Ethernet standards (thicknet and thinnet coax respectively) under older 802.3 specifications, not 802.3u. 1000Base-T is a 1 Gbps standard under 802.3ab, referred to as Gigabit Ethernet. 100Base-TX is a 100 Mbps Fast Ethernet standard that uses two pairs of Category 5 or better twisted pair cabling.

Step-by-Step Solution:
Identify which options are 10 Mbps: 10Base5 and 10Base-T clearly operate at 10 Mbps, not 100 Mbps, so they cannot be 802.3u. Recognize 1000Base-T as a 1 Gbps (1000 Mbps) standard, part of Gigabit Ethernet, defined in 802.3ab, not 802.3u. 100Base2 is not a standard Fast Ethernet designation; 10Base2 is, for 10 Mbps coax. 100Base-TX is known as a common Fast Ethernet physical layer that belongs to IEEE 802.3u. Therefore, 100Base-TX is the correct answer.

Verification / Alternative check:
Reference materials for Ethernet standards show that 802.3u includes 100Base-TX, 100Base-T4 and 100Base-FX. 100Base-TX is the most common and widely deployed for Fast Ethernet over copper. In contrast, 802.3ab specifies 1000Base-T for Gigabit Ethernet, and 10Base5 and 10Base-T belong to earlier 10 Mbps standards. This confirms that 100Base-TX is the only option that fits 802.3u.

Why Other Options Are Wrong:
Option a (100Base2) does not correspond to a standard Ethernet physical layer; the correct 10 Mbps coax standard is 10Base2.
Option b (10Base5) is a 10 Mbps thick coax standard, part of older Ethernet, not Fast Ethernet.
Option d (1000Base-T) is a Gigabit Ethernet standard under 802.3ab, operating at 1000 Mbps, not 802.3u.
Option e (10Base-T) is 10 Mbps Ethernet over twisted pair and predates Fast Ethernet.

Common Pitfalls:
Students sometimes treat any "100Base" standard as Fast Ethernet without checking the associated IEEE clause. It is important to tie the correct speed and media type to the right 802.3 extension. Another pitfall is mixing up 100Base-TX and 100Base-T4; the former uses two pairs of Cat5, while the latter uses four pairs and is far less common.

Final Answer:
The Fast Ethernet standard in IEEE 802.3u listed here is 100Base-TX.