A.I. Explanation:

Option 1: "the increase in the number of messages sent by email"

Explanation: This option focuses on the quantity of messages sent via email, which is not the main issue discussed in the text. The text does mention emails but emphasizes their impact on language quality rather than the volume of messages. Therefore, this option does not align with the main argument presented.

Option 2: "the difference between emails and traditional writing"

Explanation: While the text touches on the differences in grammar usage between emails and traditional writing, it does not make this the central issue. The main concern is about how emails affect the quality of written English, not merely the differences between the two forms of writing.

Option 3: "the effects of new vocabulary on emails"

Explanation: This option suggests a focus on new vocabulary specifically related to emails. However, the text discusses the broader implications of language change due to technology, particularly the effect of emails on the quality of written English, rather than just vocabulary.

Option 4: "the effect of emails on the standard of written English"

Explanation: This option accurately reflects the main issue presented in the text. The author argues that while emails have changed how we communicate, it does not necessarily mean that the quality of written English has declined. This aligns with the thesis that the actual use of language does not support the idea of poorer quality due to emails.