Check this cool WhatsApp trick.

Impress your friends with bold, italics and strikethrough in WhatsApp!

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• For bold: add an asterisk (*) before and after the specifics words : e.g. *hello*
• For italics: add an underscore (_) before and after the specifics words: e.g. _hello_
• For strikethrough: add a tilde (~) before and after the specifics words: e.g. ~hello~

Suggested by: Wasim Momin

More combinations :

for bold & italic  *_hi_*  or  _*hi*_

for bold & strikethrough  *~hi~*  or  ~*hi*~

for italic & strikethrough  _~hi~_  or  ~_hi_~

These new stylings can also be entered in the WhatsApp web client — but the formatting will only show up in the app-based version of chats.

Ref: Now you can impress your friends with bold, italics and strikethrough in WhatsApp

Update: Currently working on iOS and also on Android (Works with Latest Android update released on May 5, 2016)

Thanks for the Screenshot : Vishwajeet Verma